Assault and Engineer are always my two main classes. The other classes are more specialized. And in some cases, if the other team is very good with vehicles then its almost certainly an engineer only game for me.
I can mainly speak for the assault rifles since that is what I use the most.
The m416, the ACE 23, and the AEK are imo clearly the best guns. I mean I use other guns to vary it up for my personal taste but almost all the competitive players currently use the ACE 23 if they are using an assualt rifle. I can't use the gun you mentioned in your post earlier. The SAR has a crazy low firerate and I can't use guns in battlefield that have a low fire rate as I will get dusted. Personally I use the F2000 alot because I feel very comfortable with it from BF 3 but there is a bug in the game currently where you don't receive the ADS sidestep bonus speed on bullpup guns if you use anything but the ironsights.
Ak-12 is pretty good. Low fire rate gun but I can do better with it than the SAR.
SCAR-H is a pretty decent high damage AR but the magazine only holds 20 rounds. So sometimes when I use it and miss the first few shots I start to panic.
AEK has that high recoil/high fire rate but if you can master the recoil, its a greaty gun
FAMAS strictly a spray gun, hip fire weapon for me as I can't control the recoil.
AUG A3. Decent middle of the road gun. Not great. Not awful.
M16A4 I can't use burst first guns in a game like battlefield. The people who can say its very good but I don't have the discipline for it.
CZ-805 & QBZ 95-1. Haven't used either enough to say anything intelligent about them.
ACE 23 M416. Both similar in many ways. Jack of all trade guns. Decent in every aspect. I seem to do better with the ACE though.
As far as the engineer class I've only used these carbines. The MTAR, the ACW-R, and the AK5C. All good guns. MTAR and ACW-R are more spray guns and hip fire guns though.
I've also used a bit of PDW-R with that class as I heard its the best PDW in that class. Seems good and it feels more like a carbine than a hip firing machine like the rest of the PDW's.
I haven't used enough Support or Recon guns to speak intelligently towards those. I do play those classes but I generally switch to a carbine with them. I have used the LSAT and the m249 LMG's. I'm not a great LMG player but I seemed to do alright with those two.
edit:Forgot I also like the CZ-3A1 (Scorpion evo). Great hip fire spray beast PDW with a high rate of fire.