AP Varus top/mid, but top is better (you're squishy against assassins and burst mages and you have often a range advantage).
Summs: Flash + Barrier/Heal
Build:
Nashor's Tooth
Lich Bane
Rabadon's Deathcap
Void Staff/Luden's Echo (enemies stack resists/don't)
Boots of Swiftness
Last item situational (Rylai's, Zhonya's, Mercurial, Banshee's, Abyssal...)
Spell order: E, Q, W, then max W first, the second spell max depends on what is comfortable for you
Combo: R, AA (get 3 W stacks), E, AA (try to get 3 W stacks again), Q
Why is he good?
You have burst. A lot of burst. "But there are many other and safer champions with more reliable burst!" Oh really? Do they melt full tank like it was an ADC? Cause Varus does. Did I mention that proper ult does this with the whole team at once? And also that you can do this every 47 seconds with 40% CD red.?
Cons?
You are squishy and immobile, practically a snack for an assassin, plus you lose your AD Varus' range advantage.