Characters I find hard to beat:
-Squall by SeanAltly. (Attacks at spots where you're in no reach for a quick jab, if you get what I mean)
-Morrigan by hsientm (Has good AI, making it difficult to beat her. I found a weak spot in the AI though)
-WereGarurumon by Great Matsutzu, edited by me (Has hsienth Morrigan's AI coding so basically the same reason)
-Any MvC2 style characters by Kong
-Mario Bros by ShinRyoga & NeOaNkH (Has cheap AI and spams the evasion technique to close in on their opponent)
-Any Melty Blood char with AI. (They spam a lot of hits and doesn't give you the chance to attack afterwards. However, the Sion Atlasia I have doesn't have AI)
-Captain Falcon by Kamikaze (Ridiculous AI, long hitpauses on his attacks and attacks once you just about to do a simple jab.)
That's most basically it