Oh man... now that's a tough question! I'm a huge mecha fan, but I guess I never really thought of what my favorite robot is. Hmm.... I have lots of favorites, but I would have to say that the Lancelot from Code Geass is probably my favorite. I mean, that robot is so cool, especially towards the end of the series!
My personal favorite is the Strike Freedom, because of how incredibly powerful it is (in Super Robot Wars Z, it's probably the best Real Robot in the game). It has all of the strongest weapons, and even though it's unfairly overpowered, it's still my favorite.
I'll go for (Gundam 00) Setsuna's GN-0000 00 Gundam, just because it's just pure awesome : D Always when in a tight situation, the Gundam, along with Setsuna, does something completely unexpected and turns the situation, often with them acting like it's nothing special.
ZGMF-X10A Freedom Gundam fron Mobile Suit Gundam Seed and Mobile Suit Gundam Seed Destiny. I like it because Kira piloted Freedom well against many mobile suits like ZGMF-X13A Providence and ZGMF-X56S Impulse.
ZGMF-X20A Strike Freedom Gundam only appeared in a short period. If I can see a lot more actions from it, I might like and vote it.