Bride of Frankenstein
This is overall a great sequel, but it gets a bit too whimsical at times, like the scene with those weird jar people. It's also a bit too arch as well. That meta prologue with Mary Shelley winks at the audience so hard the film damn near sprains its eyelids. Still, there's a lot of good here. The blind man scene alone is just heartbreaking.
Son of Frankenstein
This one is great, too, and I have no idea why it's often ignored. The production design and atmosphere are the best of the series, and the performances and story are quite good, too. And that's saying quite a lot, considering a good deal of it written on the fly during filming. It only really suffers from maybe being a tad too long at nearly 100 minutes, and there are scenes where Basil Rathbone goes a little too far over the top, but the majority of it works.