best? L85A1... hands down... Usually if I see someone at the same time they see me, i'll only need enough time to line up a headshot, and its game over for them - unless i'm hit by a DICE bug. But, this gun also falls under "with the most cheese" imvho.
most realistic - G3. Chances are that if you see someone mid range (ie - not a sniper) then you have a very good chance of taking them out with a headshot. And up close... LOL, forget about it... just spray and strafe around them as you only use up 10 rounds, and take out a couple of their squad mates if they are in the crossfire.
after that, I love the DAO... That thing is just too powerful up close and personal... and FUN. If you roll up with this gun on a squad trying to cap a flag, well, i'll take out a squad of 3 about 25% of the time. Fun stuff.
and just to be fair, I have to include one SF weapon - L96A1... small reticle, easy to hit heads, accurate... couldn't ask for a better unlock unless of course you play at some insanly high resolution - then of course the m95