Most of the Monsters sequels are not good. Bride of Frankenstein is the real exception, though (in my opinion) the Creature from the Black Lagoon sequels are solid, Dracula's Daughter is mesmerizing, and Invisible Agent is an engaging spin-off (albeit with some unfortunate yellowface). Abbott & Costello Meet Frankenstein is entertaining and notable because it features the only other major appearance of Lugosi's Dracula, plus it's a mash-up of three of the biggest monsters.
If you want a great Classic Monsters education, I would recommend sticking with the initial entries plus Bride and Abbott & the Costello Meet Frankenstein. You can skip the Claude Rains Phantom, too, unless you're a completist.