I voted for Ultemicias castle, although it was a really really close call between that - inside sin - and northern crator.

Although i do not like ff8 I think Ultemicias castle was the best simply cause you fight lots of bosses there and do lots of puzzles and the weird music freaks you out. and once your outside of Ultemicias room you kinda dont wanna go in there in case you get your ass kicked, obviously that wasnt the case since they made another piss easy end boss

I like the northern crator simply because of the 3 last end bosses and + being able to attack one last time with either Omnislash or a normal attack in a seperate battle where sephiroth for some reason decided to be topless. But the best part about the northern crator are magic urns and the tonberrys, i remember spending hours on just levelling up

Inside sin was pretty damn cool too. You fight a load of monster which you can bribe a lot of good stuff of.