The best players to look out for are specialist returners, the ones who return both kickoffs and punts. Even better are returners on leaky defenses.
I don't know of a site which has returner rankings, so this is probably something you need to do your own research on. I would probably look for stats leaders in yards/return attempts from last year. I wouldn't read too much into TDs, they can be inconsistent year-to-year. You're basically looking for high volume returners. Check the stats, keep an eye on depth charts (via Yahoo for example), looking for players that not only return kickoffs and punts, but are relatively low down the chart at their respective position. It makes it less likely that they'd be taken off returns as to not expose them to injury.
Expect to play matchups week-to-week too. It's probably even better for it than kickers or defenses. Each week, pick a returner who's playing a team that's going to score a lot of points on them.
Return attempts is king, really.