You've probably got similar taste to me—I like a table that's a bit
relaxing to play, at least once you've got a feel for it. Black Knight (and 2000) are a lot more challenging than my usual sweet-spot, but do at least feel fair, which is why I still like them. Lots of alarmingly-fast losses when you're starting out or if you aren't really feeling it that day, but fun if I feel like being
very engaged (and after a couple dozen games to learn how to not lose within seconds, LOL)
You might like Attack from Mars, which shares the "knock down the barrier on a high-central target, hit it, move on to the next" thing from Medieval Madness. It has less fun with its theme and of the two I'd say MM is unequivocally better, but if you're out and about and run into one, might give it a try. I'd especially look for Theater of Magic, if you haven't played that one. It's not quite as clean as Medieval Madness, but in roughly the same difficulty zone and has a lot of fun shots and gimmicks, and really runs with the theme. Might enjoy Whitewater, too—it's in the same ballpark, difficulty-wise, but with a more complex playfield than MM despite looking fairly simple at a glance, but is still quite legible once you've given it a look over and a couple plays. Theme's less obviously-fun but I find it really nails a kind of kitsch appeal that works for me, at least. Both the Elvira games I've played have been in that zone, too—one's better than the other, IMO, but I forget which. [EDIT: Oh, of these, Whitewater probably has the most unfair-feeling losses when you're starting on it, just to calibrate expectations]
Oh, and find Monster Bash. Learn it. Get good at it. There are few things better in mid-difficulty pinball than finally getting the whole monster band together so they can rock :-) Fun, goofy theme like Medieval Madness.
A step up the difficulty level, I like Addams Family (doesn't everyone?) and Junkyard a lot. Everyone seems to love Twilight Zone but I can't get into the damn thing. It's got the same thing Addams Family does where if you don't have precise and well-planned shot control (looking a move ahead, if possible) you'll end up with lots of drains down the sides [EDIT: which is one of those things that feels unfair, because you can make an apparently "good" shot and lose the ball, but can be overcome if you learn the table's patterns well and keep things under tight control], but is just enough harder to learn that I've not managed to enjoy it yet.
There's another level that's easier than the ones I like, that I really despise. Like Mary Shelley's Frankenstein. I practically have to try to lose that one, to get the game to end. It's just boring and feels like it's patronizing me.