someone else in the end, which vanilla 'mech do you think is the strongest? Obviously, different situations and missions will have this or that chassis shine, but I personally think Archer-AGI to be the most OP, both in LRM and (above all) SRM configuration. I've tried the thing in the hardest missions, like stop the launch 4 and I never had to break a sweat.
My personal favourite though, must be the MAD-BH2, the Marauder hero. A 75 tonner with 1 large balistic, 2 large energy and 3 med energy + JJ. I usually deck it with a Gauss, 2 ER-LL and 3 ML/MPL (depending on level, since level 5 medium outshine pulse up to level 3). There are other, better 'mechs but this is mine. The combo of low profile, Jump capabilities to offset the slow movement and, most of all, the position of its hardpoint (Gauss on top of the model allows for sneaky sniping behind low cover) is just too much for me to ignore! And on top of that, MAD is the most sexiest chassis evar, prove me wrong!
Other honorable mentions are the humble Firestarter-9H and the awesome FS9-A (6x small energy, 2 x Medium Energy), I regularly bring the arsonist dwarf in diff 100 missions and it always performs admirably, kudos to you for showing me the light of MW5 flamers
KC-CAR is also extremely gud ofc, but I seldomly pilot it. 4xLBX10 sld is definitely the way to go imho but it makes the playstyle so boring, I'd almost prefer to drive an Anni. Great warzone chassis tho.
edit: Stalker STK-3FB is the best AI chassis, fill it with MLas, PPC and SRM6 and it still has enough for ECM, max armor and more HS than you will ever need. If you configure the weapon groups properly, I've seen the AI pull off damage in the range of 2K while only sustaining ~120-150, on average.