Here are some common levelling areas and mobs:
Levels 5 to 12
Mob factory - Focus on workers, then slimes and finally cityguard heads.
Levels 13 to 20
Canterbury - Little boy/girl, then peasants.
Midgaard - Evil align characters may want to fight cityguards.
Solace - If you have area skills/spells, you may want to try the mobs in the Inn of the Last Home.
Levels 21 to 28
Canterbury - Noblemen, then the Jester/Merchant series, then the Habersasher series.
Underdark - Good align players can level in the Underdark. There is quite a few mobs all the way from around level 15 to 30.
Levels 29 to 40
Cloudy Mountain - Start with guards, then soldiers, then lieutenants, then mage/warrior series, elite guard/virtue/anarchy, then king's bodyguard/elite personal. Confident players might want to try piercers.
Land of cult - Players with good area abilities can try the 'family' series of mobs.
Levels 41 and above
Underdark - Good align characters can kill Beholder's underling, then dretch.
Canterbury - Evil align characters can kill the pardoner/summoner series of mobs, then Merchant/Miller etc.
Hotel 81 - Go up floor by floor.
Olympus - Evil align only. You will need either detect invis or area abilites to see the mobs.
9 hells - Good align characters can kill the abishais at first hell.
Attributes & Statistics
Experience & Leveling
War & Duels