Edit 4/24/10: Updated items list to better balance damage/crit rate.
Malphite is without a doubt one of my favorite champions in LoL. Unfortunately he seems to be rather misunderstood as a champ. Looking like a giant boulder, it's easy to think of him as a super tank: High health and Defense but slow as a glacier. However with the right item build and summoner spells Malphite can actually be one of the fastest and deadliest Champions in the league. I will attempt to give a coherent description of his Abilities along with my reasons for leveling(or not leveling) the way I do.
Mal's Passive Ability and I believe the main reason people are able to use him as a tank. Early game this ability is a effective way to attack with little fear of the retaliation, Reducing all attacks by 42 points of damage at level one at increasing reduction by 8 every Level Up, not to mention any extra HP from items. Just don't forget to let it recharge once the shield gets depleted. Extremely useful, especially early game.
Mal's snare and Early Game harass. Use it on enemy champs and use it often. In Mid to Late game, correct use of this ability can mean the difference between getting a kill and watching it slip away. It can also be used as a escape mechanism as it acts as both a slow and a speed boost. As it is Mal's only ranged attack(with the slight exception of his Ult) It should be level 2-3 fairly early allowing you to hit back and escape harassers like Ashe, Zilean and Teemo.
Mal's Farming and Main Damage skill. While useful in early game to quicken the destruction of minions, this ability really shines in Mid/Late game allowing you to wipe out Super Minions and non-tank Champions alike in a matter of seconds. Brutal Strikes should be at least level 3 by mid game if not maxed.
Mal's AoE and Anti-Melee skill. A great skill for Tank Malphite as it gains more damage as Mal gains more armor. Our main use for it is it's attack speed reduction on enemies, which at level 5 effectively reduces all melee damage by half for 4 seconds; Long enough to take out characters like Teemo, Twitch and Shaco. You should have level 1 by the time you get your ult, but as we aren't stacking armor Brutal Strikes and Seismic Shard should take precedence over Slam.
Mal's Ultimate and main initiation skill. His second ranged ability and his only disable, if used in a 1-2-3 combo with Slam and Strikes you can bring a enemy down to 1/2 health before they even hit the ground. Always try to hit more then one with it and practice with it often so as to avoid missing. You should always level this whenever you have the chance.
The first item you should get is Brawler's Gloves which should be turned into a Avarice Blade and then a second. Why no HP or HP regen? Because we are trying to build a killer, and with Granite Shield you shouldn't need either of those if you play smart. And with two Avarices you are gaining one extra gold every second and combined with you runes you have over 33% critical hit rate allowing you to clear minion wave fairly quick as Brutal Strikes's AoE effect benefits from the critical damage. Next build your shoes. Depending on the situation you will should get either Berserker's, Ninja or Mercury Treads. I've heard that Berserker's Greaves are considered n00b but the 25% Attack speed will really beef up your damage output. That said if there is substantial Stuns and/or Slows I'd go with Mercury Treads. If a lot of Physical Damage enemies get Ninja Tabi, the dodge will combine with your Phantom Dancer to give you 33% dodge chance. Next start building a B.F. Sword for your Infinity Edge and the extra killing power. Now start constructing your Phantom Dancer as you need all four of it's stat boosts: Attack Speed, Crit Chance, Move Speed and Dodge. Many Games will be over at this point but if not I like to go with Warmog's Armor followed by Atma's Impaler because I can level up it's HP and Regen EXTREMELY fast at this point and the extra HP with boost your passive and combined with Atma's Impaler you will gain a extra 45-60 damage, on top of it's +45 Armor and extra Crit Chance. Now if the game STILL won't end you will need to sell your Avarice Blades to get new stuff, Stark's Furvor is great as it will boost your attack speed way up as well as giving you much needed lifesteal. A second Dancer may work as well but if you have Ninja Tabi and/or dodge runes you won't get much extra Dodge Chance as it caps at 40%
I go with Clarity and Ghost as my Summoner Spells. Malphite doesn't have a large Mana Pool so without Clarity you will either be going back OFTEN to the spawning pool for mana, or sitting behind your minions doing almost nothing.
Ghost on the other hand is my preference and really can be changed if you don't like it or prefer another spell more. However Ghost has both gotten me kills and saved my life on numerous occasions.
I go 21/9/0 for obvious reasons, To both stay alive and kill things. While the Defensive Masteries can easily be switched over to Utility, the Offensive tree is a must. I could give you all the reasons why but if you're reading the this build and don't understand why....Well you probably shouldn't be playing LoL.
Crit Chance, Dodge and Mana Regen are what I've chosen, Mana Regen can be swapped out for something different, but the Dodge Runes combined with my defensive masteries I can start with 8% Dodge and by mid game have the max +40% with a move-speed boost on every dodge. So if you chose to spec into Utility instead then the dodge can be switched to Armor Pen or more Crit Chance.
Level your Seismic Shard and get to your lane ASAP, if you are fast on your item purchasing you can get to the bushes on the side lanes before your enemy can, netting you a First Strike(Not First Blood mind you) advantage. People seem to fear being slowed by Mal so use that to your advantage to try and shove them out of XP range, as any advantage is a good advantage. Last hit minions whenever you can and Shard the squishy character(if any) in your lane, if you can get him to 1/3 health at mid field, hit your Ghost, Shard him, and try to take him down. Hit Strikes when at Melee range and hopefully you will get the kill. His buddy more then likely will come after you while you are attacking the squishy so use the last of your ghost to get out of range.
By this time you should have your Avarice Blades, Shoes and most likely Zeal. Just keep farming your lane saving up for Phantom Dancer. Once you have it you should be clearing creeps waves in a matter of seconds. If you knocked down a tower you can go ganking at this point. Look for enemies who may have over-extended themselves attacking a ally and punish them for it.
By this time you should have at least a B.F. Sword but hopefully you should have your Infinity Edge. You should be criting over 90% of the time for over ~400 damage(700 with Brutal Strike) each hit and with a minimum attack speed of ~1.5, mages should drop like flies and you should be slaying other fighters if you can get the drop on them with Ult+Slam+Strikes and then Sharding them as they try and run.
~ With this build Malphite can get away from almost anything. If worse comes to worse: Shard, and then pop your ult to knock enemies in the air while you make your escape.
~ Malphite's ult is one of the best abilities in the game for initiating a fight, using it wisely in a team-battle can single-handedly turn the tide of a battle.
~ Getting and keeping the golem buff on you Mid/Late game is essential for ensuring your mana regenerates quickly after using all four of your abilities in rapid succession.
~ One of the shining advantages about Malphite is his ability to adapt. if a DPS build isn't working out, or your team really needs a tank, Mal can easily switch item builds to fill the gap.