Hi,
I'll vote for Paladin. It isn't a full-caster, yet has something good or excellent to offer at every level, so that if you start Paladin, taking the next level rather than MCing out always deserves consideration.
Want to protect the party against some eldritch horror? Bless+Aura can provide up to a +9 bonus to saving throws for you and your friends. Want to do high single-target damage? Smite. Want to lead a horde of undead? There's even a paladin for that.
Worst designed class? I'll go with Druid, especially Moon Druid, which most druids in play probably are. At some levels, it is considered way overpowered. At most other levels, it is considered underpowered. Different classes will not have perfect consistency, but Moon Druid wins for inconsistency. The class works poorly with magic items, still an important part of D&D. The animals allowed for Wildshape will be different at every table, but that won't matter at level 10 or beyond because elementals are usually preferred. I'm not saying druids are bad on the whole, just that the class design is bad.
Anyway,
Ken