Hmm.. anything that's not small enough to fit in his mouth and anything that doesn't look like a betta! haha
A 55's a good size tank for a decent community set-up. Cory cats would look great in a shoal, as would tetras. Personal favourite tetras are pristella/x-ray tetras because they shoal really well and look lovely when they do - their subtle colours really show off and when healthy their fins really catch your eye. If you want something different than tetras, threadfin rainbowfish are small, colourful and surprisingly hardy.. and you can't go wrong with harlequin rasboras.
Keep away from anything massively boisterous... bettas are easily stressed. Maybe some form of dwarf cichlid for personality, like rams or apistogrammas? You could even possibly get a small pleco species - something like a bristlenose that will max out at around 6 inches, to keep algae at bay, or something small like otos. Yoyo loaches would also be well-suited to this size tank - they're relatively small in comparison to a few other botia species which makes it easy to keep a group.
I have kept angels successfully with bettas, however a neighbour had her betta shaken to death by an angelfish which grabbed its tail!
At the end of the day its really down to personal preference, I've only listed a few of my favourites. I've had bettas with all sorts, from tetras to peaceful barbs (such as cherrys), rasboras, guppies (the betta helped prevent a population crisis!) and other livebearers, even gouramis. But as I said it depends on what appeals to you
