You can either get AD per level or flat AD runes. I like to have a mixture of both, it's all about your play style. Boring section. :x
Some of you may wonder why I wouldn't be going for the HP mastery. Well. I personally don't like it. If you're the AD Carry, you need to be staying in the back, and since you're Varus, you've got a lot of range over people with Piercing Arrow, so you shouldn't be having trouble about being targeted. Being targeted? Kite them with your Hail of Arrows. It's very simple. And either way, the MS is a way better option because it helps your kiting. Going mid? You're going to have boots + the 4% or w/e the mastery gives in MS. Easy to dodge any skill shots, which mages have a lot of things that require skill to land. Karthus Q, Cass Q, Brand Q, okay, they don't need skill to land them, but you can dodge them easier.
~~THERE IS A TL;DR (Too Long; Didn't Read) AND A "What I end up getting," AT THE BOTTOM OF THIS SECTION, THIS IS A LONG SECTION. I can probably call this one section a chapter. Anyways.~~
Berserker's Greaves and Mercury Treads are self explanatory. Too much CC get Mercs. Don't need Mercs get Zerker's. Alrighty then.
Oh boy. The great debate I've had in my head for the longest time. (Not really, just ever since he came out.) I've experimented with all these items. (The last two items of your build) and I've just come to the decision that it's all on the situation/game play style. If you like on hit effects, and you like damage, or if the enemy has a lot of HP, Madred's Bloodrazor is definitely the item for you. I mean, seriously. That item, on Varus, is insane. It gives armor, damage, and attack speed. + THE FREAKIN' THINGY ABOUT % OF HP DAMAGE OR W/E. Lol. Trinity Force I cannot stress this at all. This item was MADE for Varus. I mean, literally. It was. Look. You get EVERY thing you need. You get the attack speed, the movement speed, the slow effect, you get some HP for surviving, and you get the on hit effect. How does the on hit effect synchronize with Varus? Simple. Look. You hit someone three times, or w/e, you throw your Piercing Arrow at them, and bam, you have the Trinity Force proc effect ready to work. Now, since you have about 30 or more AP from the Trinity Force your Blighted Quiver does more damage with the on hit effect. Sadly, not with the second effect of it, about max HP % damage blah blah. Now since you have the proc ready, that first hit that lands your first charge of Blighted Quiver will do TREMENDOUS damage. I mean, I'm being completely honest. I've tested it before, and it's completely a MUST HAVE. On to the next item. The Black Cleaver this item. It's another item made for Varus. You get three hits on the enemy, that's three stacks of The Black Cleaver, three stacks of Blighted Quiver. What does that mean? That means your Piercing Arrow will do more damage from the armor reduction. I've tried this before as well, The Black Cleaver is also great because of the attack speed AND attack damage that it gives. Next item. Last Whisper. This item it's.. It's great. I still like The Black Cleaver more, but this item is great as well. Wit's End I think this is self explanatory. If you need magic resist, get this. Maw of Malmortius is great as well, but I like Wit's End more. You'll need the attack speed, the other items in this build give plenty of damage. Phantom Dancer. Sigh. I can't believe I put this item on the "get it if you want" part. I just.. Idk. It's more of a decisive item than the first three main items. Guardian Angel is also a good item if you feel the MR items aren't enough to keep you alive. Without this, your survivability is.. A bit less than decent.
The Bloodthirster is rushed, as the first item. Berserker's Greaves or Mercury's Treads depending on how much CC the enemy has. Zeal -> Infinity Edge -> Trinity Force. These items are your CORE items. The Black Cleaver Wit's End Maw of Malmortius Last Whisper Madred's Bloodrazor Guardian Angel and Phantom Dancer are all situational/depending on your play style.
On my play style, I usually end up getting The Black Cleaver and Phantom Dancer for my last two items.
Why Piercing Arrow first? Self explanatory - it's basically your main damage source besides your right click. Lol.
Blighted Quiver maxed out second for that tremendous damage it gives.
Hail of Arrows is maxed last, but got at level 3 for both lanes. Why? Well, when you're playing and if your jungler decides to gank, at level 3, and you are still level 3, you have a slow ready, at least.
Hail of Arrows is a great spell in narrow pathways. If you can, completely block a pathway, and they cannot avoid your slow, unless they feel like turning around and try to kill you, or they flash over it or ghost past it.
Why Ignite if you already have your E? Well, Ignite isn't always used during the middle of a fight. That's when I use it. But let's say you didn't need to use it during the middle of a fight, you can use it on someone running away and you can't get them, or your Piercing Arrow is on cool down, or something. Ignite away. For mid-fight? Well, if you're fighting Volibear, Mundo, Warwick, etc. you can use it mid-fight to tremendously reduce their healing, or if another AD carry has a lot of lifesteal and you encounter each other alone, Ignite away my boy! (Or girl.)
Flash I seriously don't need to explain this to you. If you aren't level 12 get Ghost.
Varus guides need no summary. He's a bad ash. End of story.
Hope you all liked my guide! I hope you all give constructive criticism instead of just trolling away. Thanks!
05-11-12: Added Guardian Angel to the decisive/situational build.