I think speedrunning is primarily a matter of personal taste as well as considering how much time you have (a game seated at a 1 hour run vs a 5 hour run).
However, popular franchises like Mario or Zelda may be viable picks because you are more likely to have help from an active community as well as very detailed pre-existing knowledge bases if you're looking for guidance.
The only real criteria for picking a game to speedrun is that you enjoy the game quite a bit. If not, you'll probably get sick of it pretty quickly.
Actually, that might not be true in some cases. I've recently picked a game I previous hated, because many guys started to speedrun it, and now I quite like it.