For supression and general long range fights, the M249 with IR scope, Bipod and Ext. Mags. For close quarters I use the M60E4 with Holographic, Grip and Ext. Mags.
You should start using the game's own http://www.gamesas.com/topic/1318121-battlefield-3-thread-6/page__hl__battlefieldinstead of making a new one for a single question.