Hi there! I'm just your average LoL player who playsVarus. However, I've noticed the same theme for, well, every AD ranged bot. Infinity Edge, Blood Thirster, Phantom Dancer (sometimes x2), Black Cleaver, Last Whisper, 21/9/0 masteries. This got VERY boring, so I tried something. Taking a page from Teemo, I found a new way to dish out damage and wreck even the tanks.
(Please keep in mind that this build is made heavy on opinion. Every one has their own opinions. I ask that if you don't agree with my build, then look for another one instead of downing on mine. If you like my build but don't agree with a mastery or item, I'm not forcing you to use it.)

Greater Mark of Strength (0.95 ph. attack) x9
Greater Seal of Resilience (+1.41 armor) x9
Greater Glyph of Shielding (+2.7 magic resist at lvl 18) x9
Greater Quintessence of Strength (+2.25 ph. attack) x3

Physical attack marks and quintessences are NEEDED because of Varus's low natural physical attack. Armor seals and magic resist glyphs will keep you from dying for about 5 seconds, enough to go into a bush and juke or for your support to come in and save you.

Why 27/1/3? WithVarus having low natural attack and being a caster hybrid, getting 4 points in Sorcery, 1 in Demolitionist, and 2 in Butcher in addition to the usual AD masteries, last hitting, killing runners and destroying turrets are all the easier. I personally don't like the 21/9/0 mastery pages because 6 defense and 6 resist in place of easier farming and kills isn't worth it, though I do think the runes are kinda worth it. The thing is, 1 attack = 1 hp of damage, and 100 armor/ 100 resist = 50% less damage. That's useful, but that's for tanks. 6 armor will get you like... 3.0% less damage. Against the other AD carry with around 100 attack, 3 hp isn't going to be your saving grace.

Pros
+ More AD and less Caster
+ Not evenFlash will save them
+ Tank destroyer
+ Don't need Red Buff
Cons
- Piercing Arrow will do noticeably less damage
- Items do cost a lot
- Low base attack

Ways to get around the Cons:
Piercing Arrow is a very good skill, but I don't use it to initiate or during a fight. I use it to get my target low or kill them off if they run. I either use the skill's ever increasing range and it's synergy with Blight, dealing up to 15% of the targets full health!

The items. Can't really get past this. Last hit well and get some kills. Maybe don't get the full Frozen Mallet.

Runes and a Doran Blade will fix this problem, though it makes the Doran Blade a must. I don't take a point in Blight at lv 1 because it throws off my last hitting.

I usually get Heal and Flash.
Flash - Your cornered, they burnt their flash to lock you in. But wait! You book it to dragon and flash over the wall over to Red Buff, recall and they miss a kill!

Heal - This spell heals more to the caster, losing usefulness if being on your support. Plus with Summoner's Resolve, you get 10% more health! Cheat the other player out of a kill or ruin their plan to aggro you!

Other considerable spells:
Ignite - This spell can help you get a kill past turret, plus it reducing healing.

Exhaust - A personal favorite. Prevents them from getting far, fighting back and makes them easier to kill.

Promote - I've seen this work ONCE. If you wanna get towers down faster or get to their Nexus with super minions, this could work, but it doesn't see much use if you're out and about.

Clarity - NO. I know tht Varus is a AD/Caster hybrid, but with this build, you don't need to keep casting to get in that other half of your damage output. Plus clarity seems to be more of a support spell, since they're always healing or shielding you.

Mandreds Bloodrazor - You deal 4% of max health in magic damage to your target on every hit. This will wreck the other teams AD, Off-Tank, and the main Tank. Even the AP that gets Rylai's / Rod of Ages. Oh, you have armor. MAGIC DAMAGE SUCKAH!

Blood Thirster - I get this instead of Infinity Edge, trading 20 attack for 12% lifesteal, keeping me full of health and ready to duel.

Infinity Edge - 250% damage from critical hits, 25% critical hit chance. APEWPEWP- dead.

Phantom Dancer - 55% attack speed, 15% movement speed, 30% critical hit chance. PEWPEWPEW!

Berserker Greaves - MORE attack speed?! Why not!

Frozen Mallet - Hey, move slowly whilst I shoot you, okay? And look at my health. Dang, man.

This is my first build and I am very glad that it's a build for one of my favorite champions! I've tested this build and it does work. Some people might be skeptical / "durr gg varus dun no how 2 build", but if they don't lose the game for you then let them sit back as you fire away for a penta kill. Thank you for reading, leave a comment if you like it or have something to add, and vote up if you like it!

EDIT - Through a custom 1v1 game, I learned that with this build, you can even solo baron and make it out with 5%~ health!