Welcome to my not-so-humble opinions on the worst of the worst, Mechs that I'd avoid at all costs, and why. Note that all these mechs are stock unmodified mechs from Record Sheet books; I have not included any of the atrocious non-RS mechs that you'll find on MechList's "Rare" page. These mechs are the worst of the lot; anything else is an infinitely preferable. That's just my opinion, but it's based on years of game experience. Note also that many of the 3025 mechs - the LCT-1E Locust, for example - can very successfully compete on the 3050 battlefield, thanks to ingenious design, and many times they outperform their upgraded 3050 variants (AKA "the tragedy of the Grasshopper").
2750: The 200-D Hussar qualifies thanks to paper-thin armor and a main gun that's little more than an annoyance. No, speed can't save it. The Bombardier loses, as does any fire-support unit with only 6 shots for its main gun(s).
3025 (normal and revised): Besides the obvious awful wastes of tonnage (Ostscout, Charger 1A1), there are at least two junkers in TR3025. The Charger 1L barely has the armor of a Mongoose and has fewer guns than a PHawk, and the Locust 1M sports a whopping fat ONE TON of armor.
3050 IS: Time for stinky primary versions! Four of them, right in a row: Assassin, Cicada, Hermes, Sentinel - all are undergunned and underarmored. The Goliath is underarmed and low on ammo, thanks to single heat sinks, and the same fate befalls the Catapult. Ignoring the obvious Ostscout... Compared to other mechs, the "fixed" Grasshopper definitely stinks, and I pity the Dragoon pilot put in an "improved" Imp.
3050 Clan: There are bad individual mechs, like the Nova-D, Ice Ferret-A, and Gargoyle-Prime, but 3050 is notable for entire classes of Omni reeking. The Koshi/Myst Lynx is perhaps the worst offender, unable to decide whether to be a long-range plinker, fire support unit, or close-in scout. The Koshi-A holds the record for lowest damage potential (4) for an armed mech.
3055 IS: There really aren't any truly bad ones here. The Grand Titan is dreadfully overheated (though the revised version is no longer capable of a 1-round automatic shutdown). The primary Fireball is woefully underarmed, and the Snake is unsuited to its intended role, but there really aren't any truly bad mechs here. Several could have used Double Heat Sinks (Jackal, Venom, Huron Warrior).
3055 Clan: Both Jenner IICs are badly undergunned, especially the Variant, and neither is well armored. The Ghost Bear Shadow Hawk IIC variant world be good, except that it rivals the Grand Titan in its ability to instantly overheat to shutdown.
3058 IS: The Strider-A is a good electronics platform, but twin NARCs?!? There are others, but they're all of only marginal ickyness. The Longbow 7Q and OW are weak, but do well in 3025 games. The Grand Crusader is too easy to overheat, and is weak at any specific range.
3058 Clan: Anything mounting a UAC2 or LB2 or LB5 as a primary gun, or anything with fewer guns than a Crossbow-B (65t, carrying only 4 Streak SRM6s). Even those have their uses, though.
3060 IS: I applaud FASA for some excellent work here; nothing really wasted.
3060 Clan: Again, kudos to FASA on this one. For game history reasons, many of the Clan early-generation mechs are badly armed, but once again there's nothing especially bad here (the Ha Otoko is a waste, but it's meant to be upgraded to IS L2 tech.
Non-TR / Non-RS: The Marshal (Periphery) is a dreadful waste, but better than nothing, I suppose. The Colossus (Solaris 7) should've been better: a jump of only 2, 5 shots for its AC20, and twin LPLs for primary guns, even though firing either along with a secondary weapon (in Solaris combat only), even with coolant, would cook it.