Medieval Madness has the worst music of the three...other than that it wins in every single respect, and Monster Bash is so far behind AFM it's not funny. I've only played a handful of sessions on MB and I'm bored with it already.
Layout: MB has possibly the most boring layout ever (yes, worse than Wrestlemania). It's pretty much a 'perfect' fan. Every shot is a gimme. AFM is better since the shots start higher on the playfield, and require more precise shooting to nail consistently. MM has shots of various lengths, a challenging catapault shot and pop-up trolls.
Ruleset: At the risk of turning the thread into a broken record, MB is shallow. Very shallow. Kind of like South Park but with better production values. Stacking all monster modes is a challenge, but basically pointless when other strategies are easier and just as profitable. AFM has the most variety in goals. MM seems to be the deepest.
Everything else: All three games are easily explained to non-pinhead guests. All have good callouts and sound effects. MM & AFM are funny, MB less so, but still very quotable. Artwork is consistently good across all 3 titles (although AFM's saucers are kind of bland). MM's castle is easily the best toy across all 3 games.
Price: This is the killer. None of these games are good value. MM and AFM are very good games, but you can get other games just as good for less than half the price.