Lights:
Anything not the ECM variant or Hero Version of the Commando.
Raven 2X - I don't really blame people for it, 1 more energy, 1 less missile, Slower than the 3L, and doesn't have ECM.
Adders - While it maybe able to do non-ballistic builds outright better than the Kit Fox, Kit Fox can use jumpjets and it has ECM, so I don't really blame people for not taking it. Ironicly, most people I see that use the Adder get very high damage all the time due to them just ambushing constantly and people just ignoring Adders.
Mediums:
Non-ECM Cicadas -
Kintaros (outside of Champions) - I used to see them all over the place, I wonder why they aren't so popular anymore, did they get a nerf in some way and I missed it?
Trenchbuckets - I don't blame them, leveling it now and there's a lot of problems with this mech, with it being assault sized being the biggest problem. 5J and 7M is one of the better LRM boats though (hardpoint-wise and can use Jumpjets, I gotta compare it with centurion, hunchback, ) , and the 7K has some rather high mounted energy + ballistic slots. In general though, anything Trenchbucket can do, another medium can do better.
Heavy:
Dragons (excluding heros): Do you like hard mode? Play a Dragon!
Quickdraw: Not sure why they aren't so popular anymore
Catapult C4: Probably due to the lack of customization + lack of lasers.
Assaults: I see all the variants on a regular basis surprisingly.
Also, King Crab, Ice Ferret, Fenris, Hellbringer, Mad Dog, and the Gargoyle. I haven't seen them anywhere in game lately, what's up with that?