I actually aimed for level 50 for the new characters, since I didn't want to import into LF3 and have level 80 characters sleepwalking through the game. Personally, I'm not sure where the people who actually leveled to 90+ did it.
I looked it up and some people swear by the holocaust wraiths in the Arvendal Dome throne room area. Some stick with the Ice Castle, which is accessed from the Fire Guild. Some do runs through the sewers. Others stick to the East 1001 Lakes and use the Moormore inn to quickly replenish MP. Grinding the hornets in the Pachimi area using Cocka's Firestorm skill also got mentioned. I've never stop-watched them all to figure out which is fastest though.
