Sealed Demon Quest (Catch Valefor!, Marbas the Demon, etc.). (Non-Resistance)
This quest is excellent. If you do it at level 18, it gets you to nearly level 20. The demon is pretty easy to beat, then you get a free hat which has +12 of your main stat. SWEET! And the quest reward is 5200 EXP as well, which, when done at level 18, will get you to level 19 at 99.97% EXP (because you can't level up twice), 1 EXP short from level 20.
These are pretty good EXP, but may be a bit too much at level 18.
Blue Ribbon Pigs
These are no longer excellent, merely decent now. You can still train on these if you wish, but don't be expecting any quick results like from when you were level 13.