Obviously, as your level increases you'll have different spots, but the Bhujerban Mines and the Stilshrine of Miriam will forever hold a special place in my grinding heart. They were excellent places to quickly level up and get some great loot in the process. I also found them to be really cool zones, these old places filled with memories and monsters. Great places to hang out in.
Conversely, that passage to the big city, I forget its name, was easily my least favourite training spot. God damn, I hated that place. Ugh.
How about you lot? Any areas of the game that you loved fighting in? Was your training setup (in terms of Gambits, Licenses and Equipment) any different from your boss setup?