I haven't played it myself but Astroboy seems to be highly regarded. I simply don't own it yet, it is on my radar. My personal favorites are the 2 Breath of Fire games, Mario & Luigi: Superstar Sage, Drill Dozer, Medal of Honor, and Tony Hawks Pro Skater 2.
I've also heard good things about (but have never played) Lilo and Stitch, Final Fight One, TMNT, Spiderman The Movie, Ninja Five-O, and Wario Land 4
Keep an eye on Gamestop as they often do a B2G1 deal on retro games. I have yet to get a bad cart from them, and I know others hear have also had good luck with them (unless you don't have access to them, of course).
Welcome.