The best weapons in core are the melee weapons mainly cuz they one kill specials most of the time. (specially normal special infected) and as for primary weapons "AWP, GL, SCOUT/AK, SPAS, SG" then everything else comes after.
if your good with sniper, then the awp or scout is the best choice (especially the awp) but if you like something dangerous and powerful then the grenade launcher is your best friend
Is the knife good? I was under the impression it was one of the worst melee weapons. (at least in vanilla L4D2) Has it been rebalanced/changed for SS-COOP or was i just wrong all along lol