Never played more than 30 min of AC after the first one. I would have enjoyed Liberation (prefer handhelds) but it was a hot mess when I bought it.
I've not played AC1 and I've heard it had some issues, but 2, Brotherhood and Revelations were fun. The gameplay is very similar in all three though, so if 2 doesn't click then I wouldn't bother with the others.
AC3 had good intentions but was ultimately a bit of a mess. It's not an exaggeration to say that the intro tutorials take over an hour, the story was weak and the main character lacked agency. His catchphrase was "What would you have me do?" So dull.
I'm 20% of the way through AC4 at the moment and it's pretty fun so far (essentially a pirate sim with some assassin/Templar goofiness thrown in) but it's a shame you don't get to parkour your way across any big, well-researched cities.
Clearly things are swinging back in that direction with Unity, so it'll be interesting to see what (other than co-op) Ubi does to differentiate the experience from the Ezio games - if anything.
I'm planning to give Liberation a go some time, although I've heard the PC port is quite the hot mess.
That turned into an essay - sorry!