Monster Boy is pretty fun but it's much more of a puzzler than the rest of the series... in fact, almost to the point where it becomes tiresome. However, there are some ingenious level designs (especially how interconnected everything in the ghost house is) and plenty of spots where you're like "holy shit, I can't believe that worked...". The volcano area is super complicated and frustrating though.
BOF2 I cannot really recommend. I finally played this for the first time last year and it's a slow, grindy game (especially at the beginning and end), filled with tons of random encounters and dated design decisions. The battle artwork is incredible, music is OK if you like Capcom SNES music, and translation is legendary for how abysmal it is. It gets points for being a pioneer in the "church is bad" plotline department. Probably the weakest in the franchise for me (though I'm one of the very few people who actually liked Dragon Quarter
)