Reading about the various licenses I often ask myself, "ok, what the hell should I get?" It's a hard desicion to make if one isn't a "fan" of a particular system.
I don't think I've ever decided on one system to run Cthulhu with. I discovered Cthulhu roleplaying through Wizard's bastard child, Cthulhu D20. All in all, I've never had any problems with the d20 system. I find it fun and easy to use (well, after years of gaming that is). When Green Ronin published the True 20 system I became interested in it because of it's promise of being a sort of "Streamlined-d20." I didn't really get into it because at the time I wasn't looking for a generic system. Shadows of Cthulhu rekindled my interest in the T20 system, primarily due to nostalgia. I remembered all the fun I had playing CoCd20 and craved to experience it once again, this time with an even simpler system. Ironically, my problem with Shadows of Cthulhu streems from the T20 system. I simply will not spend 40 dollars on a generic system to then spend 25 on a sourcebook in order to play Call of Cthulhu. Shadows of Cthulhu would have benefited greatly if the writers would have included a 5 to 10 page sumary of the T20 rules set. Or even better: integrate the rules in the sourcebook like WotC did with Cthulhu d20. I understand that this is unfair to Reality Deviant Publishing. I'm sure the T20 license probably has a clause that prohibits including parts of the system in a third party sourcebook. Whatever the case maybe, it looks like the guys that wrote Shadows did an amazing job. Where I a True20 fan I would be all over that book.
BRP is very cool in an Old School RPG way. It's simple to run and very simple to explain. The fact that I can run a CoC game years after I ran my last one is a testament to the systems simplicity. I've never found anything wrong with the system. True, a failed spot roll can derail entire campaigns, but this is easily fixable by a Keeper who's willing to turn a blind eye to a player's bad luck. Ofcourse, Call of Cthulhu also has years and years of legendary material available which make for good sessions and good reading.
Trail of Cthulhu, the biggest thing in the indy-cthulhu world right now, fixes the whole "missed clue because of a bad roll" problem. Trail also adds the Drive, Stability, and Pilar of Madness mechanics. The game moves the timeline to the 1930s. The standard characters and setting and mechanics for Trail of Cthulhu all point at a darker angle of the Mythos than Call of Cthulhu. The mechanics for the game are pretty damn elegant too. Conversion rules for BRP makes it easy to make good use of all the resources available for BRP. There seems to be some contention about Beyond the Mountains of Madness, Chaosium's epic campaign, with Trail of Cthulhu. I've read that it is hard to port to Trail because of it's format or structure. I'll have to wait till my copy arrives on the mail to make a judgement.
Call of Cthulhu D20's purpose was to get Dungeon and Dragons players interested in Call of Cthulhu (which worked with me). The game often gets trashed on by purists claiming that it emphazises in combat. I think this is kind of stupid, considering that the group and gamemaster have sole control of the amount of combat in an adventure. Action-hungry players can turn a LARP into an orgy of death, destruction and rules-lawyering. A Cthulhu D20 ran by a group interested in studying the Mythos through deep inmersion storytelling will be able to do it with the d20 system as easily as they would with BRP.
I think Trail of Cthulhu seems to be the version that best fits my Cthulhu needs. Though I can dream (or have nightmares) about reuniting with my old group and playing an update of my old Cthulhu campaign with Shadows of Cthulhu.
I ask you, what's the best version of the Of Cthulhu series for your games?