Of them all, my favorite is Mechwarrior 4 Vengeance.  I can be found almost every night either hosting my own games or on the other gaming servers.  I primarily play attrition games and my Pilot Name is MANIKPUNSTR.  I'm pretty well know for being one of the best Mad Cat Pilots around, frequently playing against the larger multi weaponed mechs like Daishi's and Atlas's and quite often winning even though I use a smaller mech with fewer weapons.  My favorite large mech is the Kodiak.