There are many different places, i always think there are 3 important factors to getting fast melee exp:
1st, High HP NPC's to attack
2nd, NPC's with low defence, so you hit regulary
and 3rd, lots of spawns for that NPC So once you have finished killing one move to the next
The golden rule being stay in combat for as much as you can.
Spiders in the barbarian security thing have lots of spawns, low defence and a fair amount of HP, but can be quite crouded.
tzhaar monsters are pretty good too, or Pest Control is good if you join a clan.