The captured clan mechs I have (Behemoth 1, Grizzly 1, Grendel, Arcas), because the BV is way too high to field against anything but other clan players.
My Naginata, which is only good to use in big games where you can keep a stronger perimeter to avoid getting flanked
My Hatamoto Chi, because if it's not Single Heat Sinked, the BV breaches the 2000 mark, making it not worth the price.
My Akuma, because all that ammo without CASE, and the biggest gun is the MRM30, which is only useful against slow targets with +1 TMMs.
My Crusader. Again, MRM30s only work against slow targets.
Sunder - Expensive and fragile
Daikyu - I don't mind the UAC5s in multiples, but it's expensive because it has too much engine. Speaking of which...
Dragon/Grand Dragon - Too much engine for any role but in support of lighter units.
Raptor / Kabuto - not bad, but I prefer jump capability and/or C3 support.
Hachiwara - Up until recently, all the experimental tech on the Solaris designs weren't used in most groups. I've never used it outside of megamek.