Tough one, as my favorite design changes very often
00 Gundam
I like melee suit, and it looks pretty amazing too, especially with 0 Raiser equipped.
Gundam Exia
What can you say? Seven Sword!
Hi-Nu Gundam
I like the color of the suit, and it's a really pretty Gundam. Nice wing
Destiny Gundam
Dig the wings, the head, and the big ass sword
GP-03S
Hey, a very special MA look!
RX-78-2 Gundam
Of course, the one that starts it all!
Special mention:
Zeta Gundam
Gundam X
Kyrios/Arios Gundam
Wing Gundam Zero Custom