I'll second WhoKnows recommendation of the Armored Core series. I have loved AC games since the first one on the PS1.
If your into tinkering with your mech, this is the series for you. Hundreds of different parts & weapons let you completely customize your mech to fit your playing style. If you want a fast and lightly armored scout type mech, you can build it. Want a massively armored beast that can blow away anything in front of it? Build one. The variety of parts and weapons in the AC series is astounding.
Some haters may complain about the control scheme, but since Ive been playing soo long, Im used to it. Some of the newer games in the series have added simplet analog controls, but you can still use the classic setup if you like. Either way, you'll get used to controlling the game in no time.
And the best part is, most of the PS2 Armored Core games can be had for well under $20.