It is not so much the mech, but the weapons and the quirks.. Someone linked you to smurfy already, so click on the main page, and scroll down to the quirks.. You want a mech that has....
AC5,
ER large laser or larger laser
Gauss rifle
ER PPC or PPC
those are the main weapons, and one of the 65 ton mechs is probably your best bet. Of course there are also long range missiles too, which i find a lot of fun, and work perfectly well in pugs, but honestly to me their name is deceiving, as they really are not the best long range weapon. LRM's shine in the 250-400m range, where other long range weapons can work very well past that. Unless a mech is way out in the open, lrms just don't work well at the 600+ range.
So all that said, I would suggest a 65 ton heavy, Jager, or thunder bolt. the jags are slanted towards ballistic, and t-bolts energy. they have good armor and speed, and can haul a decent load out. As for armor, you pretty much want to max it everywhere, though you can scrap a bit off the legs, or head for the most part, though lights you would always max, for the rest, you can go med 40, heavy 50, assault 60 as a basic point of reference. Just keep in mind the heavy mech que always takes longer by a few mins on average so if that bugs you, you might wanna try mediums.
Other mechs, the cicada's for 40 ton mechs, the 3M for ac-5, or the 3C with a insianly quirked ERPP, with an XL-300 engine, they are almost as fast as lights, but haul a really nice loadout, and can make a great long range mech, with the speed to flank well.
as for best, really there is no such thing for the most part, though take a look at this, these are what people consider the better mechs at this point in time, but remember, this changes and next year i would be surprised if there isn't a whole new batch of so called "best" mechs
http://mwomercs.com/...yyid-statistics