You'll need to look at changelogs on
http://kernel.org/ or announcements of new kernel versions elsewhere. Changelogs are huge and can be found
here.
It is usually a good idea to stay fairly close to current kernel versions, as there are always security patches being applied - I'm sure there would have been some important ones between 2.6.8 and 2.6.12; the current version is 2.6.14.2, by the way!
If reliability is what you're looking for, you'll want to stay at the tips of the sucker tree, so 2.6.13.4 would probably be a good bet, without knowing the specifics of recent kernels. Anybody else want to comment?
HTH,
Samsara
.