Right now, if you talk about long time security inside a town, for me the safest would be Edron, Oramond, Farmine and Issavi (east side of the city, crossing the river): All them have no inside-town raids and can not get creatures lured in:
Other towns contras to leave a character afk during some seasons or at low level:
Carlin: Dryads raids
Ab'dendriel: Bugs south east of depot, and Ab'dendriel south gate raids (wolves, war wolves, wasps
Venore: Feverish citizen raids
Kazordoon: Some floors have dwarves, but most it is safe as well
Thais: Orc raids (orcs can be lured until depot)
Svargrond: Penguins can be lured in, although they are weak, maybe if you leave a low lvl character afk for 15 minutes, they will make to kill it.
Darashia: Possibility to lure creatures inside the town, and in Halloween the mutated pumpkin spawns and can be lured in.
Port Hope: Some weak creatures can be lured near the buildings of the town and there is a raid in the south part of the town of lizards sentinels
Ankrahmun: There is a raid of nomads outside the gates, but sometimes they spawn inside the town as well.
Yalahar: Bat and Vampire raids sometimes
Liberty bay: Pirate raids