I haven’t seen that many mecha series (now that I think about it, Persona Trinity Soul and Code Geass are the only ones I’ve seen so far… I once tried Gundam Mobile Suit 00, but that one is still on-hold for the moment). And if I have to choose between Persona Trinity Soul and Code Geass, then that choice is quickly made: I choose for Code Geass.
The mecha part was not my favourite part of the series (I was far more interested in the political developments and the general outline of the anime), but I have to admit that the mecha element was a nice addition. It would have been a great series without the mecha as well, but seeing those “Knightmares” (or at least I think they were called that) was pretty amazing. I even read somewhere that those have their own designers that specialize at designing solely mecha: now how epic is that?!