Jarvan IV is the newest champion in League of Legends (as of March 1st, 2011), and so far I've seen lots of people playing him and not very many people playing him well. For now, I'm going to explain, of course, my summoner spells, item build, skill sequence, masteries, and runes; in the near future I will expand upon this to include jungling, tips/tricks, escapes/ganks, etc.

Flash

Flash is still probably the best getaway/ganking tool in the game, and combined with Jarvan's E + Q combo you essentially have two flashes, provided you can get it off fast enough. Great for ganking, catching up to that last enemy, saving yourself, etc. Always a first choice.

Exhaust

I choose Exhaust for Jarvan for that last bit of time I need to kill someone. Since Jarvan is AD/AS heavy, most of your damage will come from getting those last few auto-attacks plus maybe a Q in at the last moment, which makes this a better choice than Ignite, I believe. Since the meta-game is changing (unless they change the minions/turrets again) Ignite might become more useful for those early game kills, but I find for now that E + Q comboing to an enemy, disabling them, then trapping them with your R, popping a W, auto-attacking, and then crippling them with Exhaust allows you to kill very effectively.

Ignite

As stated above, Ignite is a good third choice. Great for early game kills, or killing that quick enemy after they've already run away and there's no way you're going to catch them.

Teleport

Teleport has always been amazing, and now with the shorter death times and changes to the minions and turrets, being able to clear out a lane is more important than ever. I'm honestly not sure whether this should be a first choice or not, since Jarvan DOES have his E + Q combo that replicates Flash, but I find that you need to be getting kills and letting others worry about lanes, so far. We'll see.

When beginning an item build sequence, consider this: what does your champion's abilities scale from? For Jarvan, his Q scales from AD, and his E scales from AP. Which ability is more effective, and why? I find that E's real use is the passive bonus it grants you and the active bonus it grants you and allies, not the damage it does. Q's effectiveness, however, comes from armor reduction and high damage at range. For this reason, this item sequence is building heavy AD/AS rather than AP, since I don't find that you need it. However, AP Jarvan might very well be viable.

Early Game

I start off with a Doran's Shield for early game survivability. That small armor, health, and health regen bonus will allow you to lane longer, which is exactly what you want for Jarvan since most of his items are going to be so expensive. You cannot afford, literally, to die early game, even if the death timers are so short, now.

On my first trip back, depending on how things are going and what I can afford, I pick up Boots of Speed or Rank 2 boots of the appropriate kind (Merc Treads for MagRes, BoL for CR, etc etc, I generally go with Merc. Treads for the MagRes and passive), and a Vampiric Scepter. Next you're going to want to get a B. F. Sword to build into a Bloodthirster.

Next, You want to buy a Glacial Shroud into a Frozen Heart, though you won't be buying FR until later. The eventual huge armor bonus from FR will save you in team fights, the 500 mana is great since you have so little, allowing you to spam your skills (especially with the 20% CR), and it slows your enemies AS by 20% allowing you to out AS them easily. Pop your ult on someone, W, E, and then wail away, Exhaust if need be, and you should be able to take out most anyone at this point.

Mid Game

I would recommend a Thornmail, as well, if you're going up against a heavy AD team. If you're going up against a heavy AP team, grab a Force of Nature. Either/or depending. Anyway, you're going to get targeted, especially inside of your R, and this way you can return enough of the damage that another DPS is going to try to get you with. This way you can close off their main damage-dealer and easily take them out without taking much damage yourself, since you're going to lifesteal, cut through their armor, slow their attacks, return what they DO hit you with back as magic damage.

At this point, you should be jungling on and off if you can. You want red buff and blue buff both, (Jarvan is very mana dependent early to mid game), and definitely kill the neutral creeps when you can, but don't spend all your time jungling. You need to be ready for a gank and to save your lane, but you need the extra gold, too. If you're going to be jungling, and, really, even if you aren't, you should always be buying either sight wards or vision wards (depending on whether they have a stealth or not) for map awareness. Great ganks come from great map awareness, and you'll often save yourself if you have it, too. If you recall and you have the money, buy a few. WARDS STACK UP TO 5 NOW, so make use of it! When you have the money, recall to buy another B. F. Sword and a Dagger, and as soon as you can after that go back and snatch a Black Cleaver.

Late Game

So now you're doing massive amounts of damage, you have a decent lifesteal, and you have great armor penetration. Next, you're going attack speed. Grab a Zeal, and in turn build that into a Trinity Force when you get the chance. The passives are just too good to pass up, and the all around boosts are amazing.

There are tons of other items that would be useful, as well, you just need to know when to buy what according to the kind of team you're on versus the kind of team you're against. Banshee's Veil, as always, is a good choice if they have some snares or slows they'll try to get you with. Youmouu's is a good choice, especially for its active, but I would recommend others.

Miscellaneous

Finally, always remember to buy elixirs. Many people forget these and they can turn the tide. You want Agility and Fortitude primarily, but Brilliance can be useful if you're having mana problems, and always buy an Oracle's if they have a character than can stealth, i.e. Eve, Twitch, etc. And, again, always remember to buy wards of the appropriate kind when you can. Everyone, not just one or two people, should be buying wards. Maximize usefulness by placing them in bushes and on the curve so that you get the most amount of sight while still being able to see into the brush. Ward Dragon, ward Baron, ward your jungle AND theirs. Ward, ward, WARD!

I start with Q so that I can harass early game, check bushes, and farm easily if I have the mana for it. (Alternatively, start with E for the passive and ability to check bushes, and then level Q.) At level 2, drop a point in E. The passive is invaluable and of course the get away/gank combo of dropping an E and pulling yourself to it with Q is very good early to late game. Remember that it knocks up anything you pass through for a second, but it's not the best disable in the world. It's really just buying you time to drop an ult later on.

Drop a point in W if you need it to survive, if not, keep leveling Q then E, and dropping a point in R whenever possible and W when you need to.

This is a great DPS skill sequence, (and this is a DPS guide), but you could easily start leveling W to build tanky, if you wanted. Versatility is key. Jarvan can also jungle either DPS or Tank, so it's up to you. What's more effective for your team?

Scaling

Jarvan's has three scaling abilities, as follows:

Q: +1.10 damage per bonus attack damage
E: +0.80 damage per ability power
R: +1.50 damage per bonus attack damage

What this means is that, with your full build, and at level 18, with these masteries and runes, you should be able to pull off something like this: Q for 26% armor reduction and 396.5 damage (base 270 + (115 bd * 1.10), then R for 672.5 damage (base 500 + (115 bd * 1.50), then auto-attacking at ~1.9 attacks per second with an AD of ~165 while stacking up to 45 armor reduction and stealing 15% of that back as health with a 30% chance to crit for double that (~800-900 damage give or take armor, etc), then Q for another 396.5 damage.

A solid 21/0/9 build is great for Jarvan. In offense you'll get improved exhaust, increased damage, armor penetration, critical strike chance and damage, etc, and you'll get experience bonuses, health and mana regen, and lengthened neutral buffs from utility.

You don't need Smite to jungle with Jarvan, so I would recommend against putting a point in Plentiful Bounty in favor of an extra point in Sunder, for armor penetration. If you think you need it, though, by all means, try it out. Smite definitely makes jungling much easier.

For Marks I choose even more armor penetration, giving you 15 armor pen. at the beginning of the game and stacked on top of Black Cleaver's passive, you have quite a bit. (That's why you don't need a Last Whisper.)

I choose mana regen Seals so that I can lane longer early game. Even late game the mana regen is very nice since I don't bother with getting any mana regen items with Jarvan. Most of your damage comes from auto-attacking so there's no reason to worry about it. Mana shouldn't be a problem if you're using your skills wisely. (If it is, pick up an elixir of brilliance, if you need one.)

Attack Speed Glyphs have a better bonus than marks or seals of the same kind, so I choose these. Yet again, damage here comes from auto-attacking, so you really want the AS.

CR Quints are a great choice for early game getaways and shields, and they help you late-game since the only room for CR you have is Frozen Heart, and while 20% is nice, 29% is better. Add about 3% from your masteries and you're at ~32%. If a support on your team grabs a Soul Shroud, you're set. If not, blue buff is always an option.

Jarvan's passive, Martial Cadence, deals 8% of your target's current HP as magic damage. Once used on an enemy, you can't repeat it for 6 seconds. Make use of this by quickly targeting multiple enemies or creeps to do large amounts of damage and then focus on the ones you can now kill. 8% might not be a lot for a squishy, (though it's enough to make a difference), but that's a hell of a lot for a tank. Keep that in mind.

When farming, target each creep once and then either one-two-last-hit them (you should be able to do this with one or two hits by late-game) or use your Q to take out a line of them at once.

All in all, Jarvan is a great DPS that will soon be a force to be reckoned with. Remember, play smart, don't overextend, don't overestimate yourself or your team and don't underestimate the enemy. Map awareness is key to great ganks, etc etc.

I hope you liked and do well with this build guide. If you think something should be pointed out, something should be mentioned or discussed, or something is wrong or could be better, please comment! Post results, I'd like to see how it's treating you.

3/2/11: Changed Berserker's Greaves for Merc. Treads because of lack of magic resist and BG's AS isn't significant.
3/2/11: Rearranged items for survivability and increased armor early/mid game.
3/3/11: Changed to Jarvan.
3/4/11: After taking things into consideration, decided to change to Trinity Force instead of Phantom Dancer. Much better, imo. Reordered the item sequence to match.