The best Mechs to use (once they are available) are as follows:
Atlas Bushwacker Cyclops: Has the best sensor range of any Mech as well as an impressive weapons load. The best mech in a group. Mad Cat Men Shen: Preferably used in the near beginning of the game as a sensor and recon Mech. Probably the heaviest mech with ECM suits. Razorback: Use these at the beginning of the game as sensor mechs until the Men Shen is available. Shadowcat: Good fire support and escort Mech for larger Mechs. It is the lightest mech in the game other then the Hollander to be equipped with the a Gauss Rifle as a standard weapon. Its advantage over the Hollander is that it has more weapons for shorter range combat, while the Hollander just has the Gauss Rifle and can easily be disarmed. Thor: Has a good weapons load for short, medium, and long range combat. Vulture: Good choice as a missile Mech, second to the Mad Cat.