The T-51b Power Armour is my personal favourite, it just has that walking tank look about it, in addition to the bad-ass appearance.
Of course tactically I wouldst want the Advanced Power Armour Mark II.

T-51b is god-tier. (Especially so in Fallout 3, so damn pretty.)

Power Armour form Fall 1 is best. Advanced PA from F2 it's nice too but Fallout 1 was first and that PA look's less s-f & more in fallout art-deco style.

Advanced Power Armour all the way the Enclaves armor just has that look like ill screw your stuff up

I like the Outcast armor from Fallout 3 with its black and red color scheme. Also, the Tesla armor. I played Fallout 3 first, so I think I might just be biased, though.

My favourite is the Enclave armor, the Hellfire helmet looks very downs-y to me, and don't even get me started on the Brotherhood Of Steel (as in the PS2 game, not the organisation) power armor.

I like the T-45d PA from Fallout 3. It always struck me that the other types of Power Armor were a little off in their body proportions, by which I mean that you'd have to dislocate both arms in order to fit into a suit of T-51b. Plus, it has that rugged, rusty look which is just so perfect for the Fallout setting. The western Brotherhood's armor is so shiny, you'd never realize that they go out and fight in it.