This is the hardest "Battle of the..." yet for me xD
Talos was a hub, a crossroads. The Chicago of Paragon City, and as such I did spend a ton of time there. Talos even had one of my favorite spots in the game, a small island far north with no mission doors, mobs, or any reason at all to go there, but I truly enjoyed that little secluded spot. I met someone who became one of my best friends, in game and out, in Talos.
But Croatoa was fun. Croatoa was one of the first times where they did a story arc that spanned across an entire zone, allowing you to get through a decent number of mid game levels and not just be "finding the painting the Freakshow stole" for the zillionth time. Sure, late in the game I didnt go to Croatoa often, but I always enjoyed it's unique mobs, various giant monsters, and the eerie beauty of the place.
That's a really tough call.