One only...ok.
Well ... okay.
MM1: Yellow Devil. But the choices are rather limited.
MM2: Gutsdozer.
MM3: Can I say Doc Robot? Technically he's not a fortress boss, but he is a post- 8 standard stages boss. The actual fortress bosses don't really stand out to me.
MM4: The first Cossack one was pretty challenging when I was a kid, so I'll go with that one.
MM5: Maybe ... Sentinel 2? I like how he ferezes you.
MM6: None stand out in particular.
MM7: For looks, I'd say the flaying oni mask, but I always die on that guy. Don't like him. So ... pass.
MM8: Played it quite some time ago ... pass.
MM9: No wii = no MM9. The Devil Blobs are kinda neat. But I can't really say.
The RMs are my favorite part of MM. The Wily stage bosses don't really stand out in my mind, except for a special few. So saying some are my "favorite" is ... difficult.