Lion
It's certainly much faster or seems to be (core i7 , 2.93 GHz, 8GB), although I don't much like the front end, and have tried as much as possible to make it look like 10.6. There are some problems with what comes with Lion though. I think there are big issues with Safari 5.1 and Lion - I am getting lots and lots of "safari can't load page" etc, and misloads of pages where the page disappears to the bottom right hand corner of the window, and may or may not reappear from the top left hand corner. I am still having issues with airport and it dropping my wi-fi connection. You should check if the progs you know and love are compatible with Lion, many simply won't work, or will only part work - fixes often not adequate. Be prepared to dish out more on progs if you are one of the unlucky ones.
I did post something about this on the Apple support website, but the post was refused because the moderator said my comments were ranting! Clearly Apple and free speech don't sit well together. Maybe they didn't like my comment about increasing popularity over Windows, and possibly the problems were down to them employing a few ex Windows "kids" to re-write Safari. Yeah must have been that 🙂
Ah well !
Jon