I really enjoy Rising Tide. It's my favorite Pandemic iteration thus far.
Unlike Iberia, it's different enough that I don't feel like it's just another slight variant of the same game.
I'm a pretty big Pandemic fan though and have no problem with owning all of them, so take that into account I guess.
I love Pandemic as well. I caved and just ordered everything. Wife is gonna kill me, but it’s easier to ask for forgiveness than for permission.