Colossus X is generally considered the best armour available in Mass Effect. There are others with superior biotic protection, but their other stats are so low in comparison that they still fall behind.
Also, if you're bothered by appearances, Colossus armour looks pretty cool too.
Colossus X or the Armax Arsenal Predator armors (NOT the Hahne-Kedar ones - the Armax armors have a L, M or H to denote whether they are Light, Medium or Heavy). Colossus X of all sorts and Predator L X are relatively easy to find; Predator M and Predator H are very rare, and you probably won't find them until your second or third playthrough.