1. Life
I think this is pretty unarguable, although I'd like to see people wait a few months to use the b-word. Currently Life is in the Ro16 of GSL, Ro8 of SSL, 5-3 in proleague, and has been in the finals (and won 2) of his last 3 weekend tournaments. I really don't see any debate to be had for #1.
2. Maru
Even before his IEM Taipei run, I think Maru deserved #2. 4-3 in proleague and Ro16 in both leagues is good by itself, but Maru, unlike some newcomers, has shown the past consistency necessary to maintain a top 2 in the world spot and is the least likely to just be on a hot streak. The 2nd place finish at IEM just confirmed that Maru is for real in 2015.
3. INnoVation
Undefeated in 2015, the Machine of the Battlefield. Innovation is only #3 and not #2 because he's not in the SSL, having lost to Stats 3-2 in Challenger. However he got 1st in his GSL group and is an amazing 5-0 in proleague, on top of being the reigning GSL champion. It's entirely possible he's better than Maru, but Maru's results look better right now.
4. Rogue
Much like Life, Rogue is in the Ro16 of GSL and Ro8 of SSL. However, unlike Life, Innovation, and Maru, Rogue has no real past results to speak of, making it unclear if Rogue is really the next great Zerg or whether he's just not figured out yet. He is 3-1 in proleague, but losses to Fantasy and Myungsik don't exactly inspire confidence, even if he did beat both players in the deciding matches.
5. Dark
A Zerg that's getting even more hype as the next best thing than Rogue, mostly due to going 5-2 for SKT in proleague. The former Slayers b-teamer is also in the Ro16 in both GSL and SSL. Someone in an LR thread pointed out that Dark plays weird and might just not be figured out yet, but I'd say that what Dark, Rogue, and Life have in common is that they can all switch between weird games and solid macro games in a very unpredictable fashion. This is what seems to work in the current meta and more predictable Zergs like soO have fallen off.
6. PartinG
One of two players who got bumped up due to IEM, and the highest ranked player not in proleague. Parting has shown excellent PvT (beating Maru, a great TvPer) and PvP (stomping his former teammate Classic) but didn't have the PvZ needed to get past Life at IEM, or Dark in SSL. Despite that, he has a recent premier tournament win, which is enough to get him the #6 ranking.
7. Soulkey
The other player that got bumped up due to IEM, and another player not playing in proleague. Besides doing well at IEM, Soulkey is also in the Ro16 in GSL. His only recent losses are to rank 2 Maru in IEM and to rank 4 Rogue in SSL. Soulkey is easily the best "safe macro zerg" right now in a meta that doesn't seem to be favoring his playstyle.
8. herO
Before IEM Taipei he was the most recent premier tournament winner, and at IEM Taipei he lost to the eventual runner-up in Maru. In SSL he was knocked out by the current #1 on this PR, Life. I'm a bit of a herO fanboy and I'd really like to rank such a likable player higher than 8th when he's only been knocked out of tournaments by the top 2, but a 3-3 record in proleague doesn't really inspire much confidence in herO right now. Also he appears weak in PvP right now, losing to (but not getting knocked out by) Stats and Terminator.
9. Stats
I didn't want to rank Stats so high, but the, ahem, stats, made me. Sorry. He's 3-1 in proleague. He beat #3 Innovation in SSL Challenger and #8 herO in the Ro16 of SSL. His major weakness is his failure to qualify for GSL. Stuchiu was right, Stats is a top 5 (probably top 3 at the moment) Protoss. He had a mostly unnoticed Ro8 GSL run last season, and is now in the Ro8 of SSL, again quietly.
10. Rain
Rain doesn't seem like anything special, he's only in one of the Korean leagues and doesn't play in proleague. However, the reason Rain cuts it for top 10 over the CBNCs, besides that he was the runner up at IEM San Jose, is that Rain beat Life, and unlike herO and ForGG, Rain didn't lose to Life later. He beat the best player in the world, and the best player in the world hasn't gotten his revenge. (yet?)
Close but no cigar:
TY and Dream. This is a combination of TY's meh proleague record, Dream not having been sent out much in proleague yet, and both of them having yet to play their SSL group. Also both are somewhat untested. They beat Yonghwa and Bomber in GSL, and Dream beat a slumping Solar to qualify for SSL, while TY beat Scarlett. Every player ranked 6-10 has beaten a top 10 player in a Bo3 or more lately and/or has recent results, while TY and Dream have had weak opponents and are newcomers. If one or both of them advances over Dark and Maru, they will be a top 10 player. Dream is potentially top 5 if he advances in SSL and gets a few more proleague wins.