Fidelias said:
Anything that is not Mechwarrior is wrong.
But in all seriousness, I don't play a lot of mech games. I do love them, but I can never seem to FIND any. They just aren't put on a lot of commercials and such like other games. I only really played the Mechwarrior games and Armored Core games, and Mechwarrior was much more innovative. But I don't really know.
Mechwarrior's gameplay hasn't really changed much over the last
umpteen iterations (my experience only goes back to MW2: Mercenaries). Armored Core on the other hand changed around the 3-4 area from a slower control set to super fastpaced combat. They changed up the formula slightly and both still work (though switching from ACFA to AC2 is one hell of a jarring sensation).
Zone of the Enders is an arguable innovative game, as that was the first game (localised in English that I could play >_>) which had really proper high speed free flight mech combat.
I have, however, seen that there are numerous Gundam games in Japan which never get translated or released over here. Some of them could be more 'innovative' and 'original' but I've never played them since I don't speak foreign.