40K with no contest <_<
WH40K, but i think that's because i know so little of it.
All you need to know is that everyone is fucked forever, except maybe the Tau.
Except not, because there is some fucked up powerful shit hiding everywhere in that universe.
The Tau are just as fucked as everyone else really >_>
They only seem slightly less fucked because they haven't had to fight Chaos, Daemons, Tyranids, Eldar, Orks, Orks, Orks, More Orks, Necrons, Dark Eldar and the like simultaneously for the last 10,000 years.
GW broke their own universe with the multi-fleet Tyranid stuff, the idea that there are multiple galaxies out there with countless hive-fleets all drifting towards this one is a bit too OP because they haven't introduced a proper counter to them (Like the Eye of Terror and the Pylons) so unless they give the Imps some sort of mutagenic virus that works 99.9% of the time on gribblies it's going to be Game Ogre for everyone >_>
I think the other reason they are kind of left to it is because it's an enemy of my enemy type of deal they have with the Imperium. Soon enough though, they'll take one world too many and Bam! Damocles Gulf Mk 2.0 is going to be launched and I don't think the Tau will be able to escape a full on meatgrinder assault by the Imperium.
At the moment though it would be too costly for the Imperials to actually go for the Tau in response to a handful of colonies being turned into Gue'vesa factories.
Sooooo yeh, 40k would definitely be the most interesting for me <.<