Way too many awesome designs for me to choose a best design so here's a bunch of my favourites:
I love the design of the Brotherhood of Steel power armour design:
It's imposing and somewhat terrifying. I also really like the gas mask style helmet.
On a different note:
Really well designed, the animations especially are sublime.
Garrus Vakarian:
This guy in general is just a complete boss.
Then there's always this badass:
Actually, I changed my mind. The Chief will always be my favourite character design.