Forest for the quick runs (and in the case of a Redria, fun rares to find

)... Ruins for Falz, of course... but I actually like Mines 2 a lot as well. It really gives off that sort of "military base" feel - it's pretty cool. Mines 1 and Caves are just annoying though.
For Episode 2... you just can't beat CCA. I particularly love the mountain area - the beach is nice, but if it had the kind of sky/color/sunshine/atmosphere/whatever that the mountain has, it'd be cooler IMO. Jungle is decent, but not my favorite, although that one place in Jungle East with the railing reminds me a lot of a zoo exhibit. And the CCA itself is pretty cool too, with the overlook of the jungle - also sorta like a zoo exhibit.

As for Seabed... some of the most annoying enemies in the game hang out there (curse you Sinow Zoas!!!1 :ermm: ), but it's not as bad visually (and musically) as, say, the Caves, which is the most boring, pointless level ever created (and if it weren't for De Rol Le, there would be nothing fun about that level at all).