Most of these have been stated before, but I will throw in my favorite three builds. The play styles of each build are quite different, and are key to your success.
2xPPC+Guass. I recommend using one 'S' side torso for jumpjets.
Poke around, snipe, and stay off the front line. You can stick everything in one side of your mech, so learn to torso twist between shots to increase your longevity.
Typical Laser vomit. 2xLPLAS+4xMLAS I don't use the TBR-A ST, as I dislike the negative quirks. This is a frontline/2nd row mech. Its very much a jack of all trades build. You can poke, brawl, or support with this mech, but the key is making sure you know you cant outbrawl a splat build, or outpoke a PPC Guass set-up. Pick fights you know you can win, Take good trades, and make sure you are fighting smart.
My favorite, and the one I will actually be bothered to go into detail with.
4xA-SRM6+4SPLAS+2SLAS
http://mwo.smurfy-ne...a1820e532cdc145
Proper success with this build requires excellent map awareness. Don't be afraid to go off a little on your own for some neat flanks. Once you are practiced enough reading the mini-map and guessing at the enemy teams location, you can get some really nice flanks right into the back of the enemy team. I usually wait until combat has started in earnest for about 30-40 seconds(This ensures the entire enemy team is looking in the opposite direction from me), then I roll right in from the side/rear. Patience is key, as you can kill most mechs 80 tons and under with one well aimed alpha into the RT of the mech. Nothing makes you feel more bad *** then killing 2-4 mechs in under a minute with a well timed flank. With this build you can out brawl almost any mech in the game. Be sure to take double cool shots for this build.
I fell in love with this build after getting 5 solo kills flanking the enemy team in the tunnel on crimson. I killed a kodiak, a black knight, and a HBK IIC before the enemy team even turned around. Finished off two more when my team rolled in from the front. 'secured' 3 more kills afterwards for my first ace of spades. Once you have moments like that using this build, you will never want to run anything else. Also, you feel so dirty when you core out a mech from the rear before they can even react.