Light Mechs:
Adder. I've never used this one, either on the tabletop or off it (but that will change come march), but I do really, really like it. It's a light mech with twin PPCs, how can you not like that? But more importantly, it also looks good, particularly the new plastic one, echoing a lot of the classic clan designs like the Dire Wolf and Timber Wolf, but still being it's own thing and not some weird, tiny mismash of parts (cough mad dog cough).
And also it has two PPCs.
Urban mech. Also never used it on the tabletop, but this was my light mech of choice in mechwarrior 4. Turns out, when you strip out all the guns, mount a heavy laser, more armor, and a slightly bigger engine, it's a blast to use. The TT model is kinda horrible, but the clan one is alright, and I'm interested in seeing how the new plastic one turns out.
Honorable mention: Arbiter. Not because it's good, because it's not, but because the idea behind it is awesome and cool, which is also how it looks.
Medium:
Raptor II: Another one for the "never used, but like the idea" pile, a stealth mech sounds really, really neat, even if that stealth comes at the cost of firepower.
Uziel: A nice, solid all-rounder, doesn't really seem to excel at any one thing, but it also doesn't have any glaring weaknesses either.
Honorable mention: Ursus and Ursus II. While both mechs are fairly neat, the Bears very obviously stole them from the Coalition States, so I don't think they really count.
Heavy:
Oh yeah, now we're talking.
Mad Cat MK IV: Fast, tough, packs a serious punch in any configuration, and most importantly, looks much, much cooler than the MK1. Yeah, I said it. Only downside is the rear mounted SRM6s don't get much use, but I prefer to think of it as instead getting two SRM6's worth of open pod space and weight to play around with.
Lament: I'm starting to notice a trend of loving any mech that mounts twin PPCs. Not sure why. But yeah, the Lament, presumably named for what it makes the enemy's women do, is a fairly solid mech, with the only real downside being it's dependance on the Radical Heat Sink.
Catapult: Well, there is a two PPC version of this, but I don't run it. There's something very nice about the 'pult, it's a cheap, basic missile carrier and you can fit a pair of just about anything on it without too much trouble. That's nice, there's a lot to be said about designs that have an obvious, simply but effective tactical role.
Gallant: Another simple but effective design, it's hard to screw up "big gun and a bunch of missiles", so when you're looking for a nice frontline mech, you could do a lot worse.
Honorable mention: Penthesilea. The prefered heavy mech of strong independent women who don't need no man, and a pretty good looking one at that. Named for some lady from greek mythology, and unless you have a degree in that, impossible to actually pronounce. Hence why I've ended up refering to it as the tumblrmech.
Assault:
Mad Cat MKII: See the MKIV, and then make it even bigger, even meaner, and even cooler.
Fafnir: I run the version with improved heavy gausss rifles. Because it's nice being able to make a hole the size of a light mech in a light mech.
Kodiak II: The most Ghost Bear mech in all of clan Ghost Bear.
Viking IIC: I usually end up taking the Catapult with various gimmicky missile builds. This one, on the other hand, is the one for when you want to get serious about missile spam.
Honorable Mention: Trebaruna. See the Ursus, but they jacked this one from Zoids instead of Rifts.
Plus it was one of the earliest designed mechs according to the lore and really lends to an effective looking design on the surface of it all.