There's the Percentile system used by the Chaosium game, and then there is the D20 system used by Wizards. Is there a "superior" system for ultimate cosmic horror role-playing? Am I forgetting another system?

I think each system has its own merits and flaws. I haven't used Chaosium in ages, but I seem to remember it being confusing and unweildy. It is, on the other hand, specifically geared towards the mythos. D20 is flexible beyond words, and has the added advantage that it can be incorporated into other games with the same mechanics. I always liked the idea of a chaotic evil Cleric of Nyarlathotep myself... But that system comes with all the flaws of D20. I don't think you're missing any other tabletop systems.

http://www.google.com/Top/Games/Roleplaying/Genres/Horror/

I'm sure there are more horror games, but Call of Cthulhu and Dungeons and Dragons are likely the most popular and the best for playing horror. It's a difficult thing to do though.

i doubt if anyone will ever see this but there is a licensed game entitled realms of cthulhu, if i'm not mistaken, coming out for the savage worlds system, which was created by pinnacle entertainment group. it may be out now, or it may not. but it was causing quite a stir on pinnacle's site a few months ago.

I've played both and I own both. Here is my critique.

D20 CoC is entertaining if you are more of a combat oriented player. Because of the fact players can easily beat the monsters with the right planning and level, I don't really like it. I mean if me and my friends who played waited till all 5 of us got to level 30-40 we could take on a good number of the Gods in the book. It just seems way to primitive and I think it failed to capture the true lovecraftian essence.... HOWEVER!
If one was wanting a lovecraftian RPG I would suggest trying the D20 CoC first just to get a bit of a background in Lovecraftian Role Playing.

Chaosiums CoC
I think it's the better of the two, it's more complex, realistic(sp) and difficult. It would take investigators a long time and a lot of blood shed to take down the average monster in Chaosium's book, and it's just full of awesome! It's way more focused on actual role playing and woe unto the ivestigator who runs into guns a blazing! Cos as Grandpa taught... It doesn't work... And to also add onto the RP experience of the Chaosium CoC try playing it without leveling your character....

Because the Chaosium CoC is more complex I'd go ahead and encourage the aspiriing RPGer to try the D20 first.

Having used both, I have to cast my vote for Chaosium. D20 is simpler, but in this case, simpler is relative and not necessarily the best way to play HPL's mythos. And, if you haven't tried playing the original D&D in a while, then take a look at it and then tell me it's "less complicated" than Chaosium's CoC!

As someone else points out, using D20 would allow you to turn a CoC campaign into a "Monty Haul" version of HPL's vision, but using Chaosium's CoC forces the players to really think and roleplay, since not only are guns deadly, but sanity loss is a real risk.

Plus the sheer quality of Chaosium's design work and scenario packs is a thing of beauty. All things considered, I prefer "classic" CoC versus the D20 design.

I did take a look at "Realms of Cthulhu" but after spending over $20 for it, I found only ONE good idea in the whole thing -- the rest just adapts the CoC system, which doesn't really lend itself that well to adaptation.

Finally, there is "Trail of Cthulhu" which adapts CoC to the "Gumshoe" system. Really the only innovative part there is that in "Gumshoe" you always find the clue you need! While that's good for some clues, it's also nice to have the players proceed with a few gaps in their knowledge -- especially if you can make them suspicious there might actually BE such a gap -- it tends to increase paranoia and fear -- both of which are sort of crucial to a good CoC campaign!