The best for me is the Masakari, decent speed for 85 tons and very good heat dissipation. I equip this one with nothing but energy weapons, two ER PPCs, 2 Large Pulse lasers and 2 Medium X Pulse lasers. Along with equipment like AMS, etc... The pulse lasers alone can do good damage while in close range and when used together with the PPCs, all it takes is one good aimed alpha strike and afterwards about 3 seconds of pulse lasers, any a mech is surely to fall.
Favorite mech has to be the annihilator from mechwarrior 3 or Uziel with two PPCs and three srm 5s. Has good speed jump jets and packs the power to snipe.
mine would have to be the Thor, (or Summoner, if you prefer) great weaponry for it's size, and it's very fast and maneuverable for it's size. and i can load it out with pretty good weapons for whatever i need to do... nothing better. if the Daishi was faster maybe i'd like it over the thor....... but not till then.
Well the madcat is one of the most ackowledged mech's and it is quite the fighter, especially if you include some of the later variations like the mark 2 and mark 3's
however i wuld like to point out that a Ryoken 2 with jump jets is a great contender in any fight
and the Jupiter Assault mech can NEVER be counted out in a fight.