UAC20s when the server decides to have hit detection work.

SSRMS - Same as above.
All laser varients, because they never seem to have a problem dealing damage, and I've always liked lasers. Though I seem to be less successful with Xpulse lasers than I'd like, although there are too few varients with them.
MRMS - Same as SSRMs, though the bug with MRM20s bugs me. I really like the spray of missiles you can put out with something like the MRM catapult though.
LBX - Nothing makes me happier than instagibbing aircraft when they think they have an easy target.
(C)Gauss - Nothing's better than a simple railgun weapon. CGAUSS is even sexier.
Honestly, I like all the weapon systems. These are probably the ones I use the most though.
I have the most TROUBLE with:
ATMs - Always seem to get too close and they bounce off.
LRMs - Similar issue with ATMs, though I seem to just not do well with them. Occasionally I absolutely rape with them, usually I ignore them.
Heavy Small/Heavy Medium lasers - Can never get the massive effect out of these that other people seem to. Case in point the scalpel puma varient - have been raped by it, can never get it to do much for me though. Prefer the Puma with the 6CERMBL myself. Heavy LARGE lasers I do fine with though. Love the Thor B.
(U)AC10s - Think they just need a buff. Have tried a few assets with them, died miserably, never went back. Hoping for a buff in 0.5.0. The only asset I've had luck with that fits these is the Shiva with 4 of them mounted.
HGAUSS - Never have much luck with it. Will probably try it out when the fafnir hits again.
AC2/AC5 (not ultra, not clan) - I hate, hate HATE how slow the non-ultra, non-clan versions of these fire vs their damage output. Find them utterly useless.
RAC2 - Pointless damage, only on a couple assets anyways so no big deal.
All that pretty much leaves is a few minor weapon systems, like PPCs and ltGauss, all of which I enjoy, but not to the extent of specifically seeking out assets fit with them. Also left out the longtom, but that's an asset in and of itself, so...
After all that, if I had to pick ONE weapon system that I'm best with, I'd have to say lasers. Easy to use, good consistent damage, and can kill Mechs, Tanks, ASF, Vtol, and BA all equally well. Of course, specialized weapons are better in certain circumstances, but lasers are by far the easiest to win consistently with.