I'll tell you what I find good, use that information to your advantage.
AC20 Osiris - this mech is simply too good to pass up, I'm sure it'll be toned down in the next version
twin gauss Vulture - now at a nice price point, with ECM, it's a great sniper
Nova Cat, any of the all laser variants - best if you get these early, very good vs light mechs ^^
Mad Cat prime - BAP, AMS, heavy armor, heavy firepower, this is what I end up in most of the time (but it has about 10 tons less equipment than any Nova Cat variant, which means it's lacking heatsinks, so I can sometimes damage myself when I run on the edge of the heat scale - which is all the time)
Sometimes I use:
AC10/MRM Warhammer- very cheap heavy class mech, ECM and good armor, it doesn't work every time as properly driven light mechs can evade most of it's attacks
Pulse laser Warhammer - I sometimes get this, it has ECM and makes a good substitute for the pulse laser Nova Cat
AC20 Shadowcat - a fast mech with a strong gun, it runs hot though
That should be about it.