mw2 is one of the best i think. not the best graphics or most well designed, but it has mech gameplay second to none.
seconded, Mw2: Mercenaries was the best of the three mechwarrior 2's, especially the titanium edition with the XP Patch.
Allegiance is a fun combat game, by the way.
And mechcommander 2 is a good strategy game.
and then... there is Mechwarrior 3. It has the atmosphere that is similar to Freespace, FS2, and Wing Commander: Secret Missions (we're doomed, outnumbered, outgunned, etc.) funny thing is, in MW2:Mercs you could buy your mechs or salvage them, in MW3, you were COMPLETETLY dependent on salvage, so the thing was not to destroy the mech, but to cripple it by shooting trough the cockpit (Kill the meat, save the metal), or blowing the leg off. Unfortanly, it doesn't work well in XP, and i do not have the spare space for a partition with Win98 on it.
And now, I am off to play MW2: Mercenaries!