Morrigu = Rape machine, holy shit. It kills all ground assets very quickly, but it's kind of helpless against air and competent BA players. It has hilarious handling, especially when going downhill; High center of gravity + big turret on the rear of the tank = fishtailing when going downhill.
Hephaestus = Good scout with good armor, but it lacks the Harasser's sheer speed and firepower. More suited for support (AECM variant is pure support, it only has 3 small lasers)
Thor = Fun mech to use, but it's kind of feeble at defending itself. The low max/minimum torso elevation really hurts it; enemy mechs can hug up against it and the Thor pilot can't see them; or shoot back.
X-pulses = Fun to use but they're really really terrible if you get low FPS, like me. Easier to use than regular Pulse lasers though.
ATMs = Kills anything without LAMS very quickly, but LAMS make them basically slightly more dangerous than SSRMs. The minimum range keeps it from being too overpowered but the hard cutoff is pretty annoying; 160 meters it's extremely deadly but at 150m they're totally harmless
Heavy Lasers = Pretty overpowered IMO. They make more heat but Clan assets usually have like 2-4x more heatsinks than most I.S. assets due to their DHS. Their range is equal to regular I.S. Pulse Lasers, but they do ridiculously insane amounts of damage. The Puma that boats heavy lasers can leg a mech in two or three alphas.