Ousawa

A ton of 'em. Though not a lot of them are good, Snakewood, Dark Rising and Glazed instantly come to mind if we're talking about GOOD ones.

Ash Gray. FireRed Omega are two I can name off the top of my head, there are a few to be fair/

I believe TheKingNappy is playing through a ROM hack called Pokemon Sweet at the minute and that looks decent and slightly challenging (but not as challenging as Reborn though :|)