HB zone (not the citadel tho, screw citadel honestly with all the deep corners ppl used to hide)
reasons:
1) only 78+ people with quest could enter, so no annoying lvl 1 cameras on every 2 steps monitoring your movement
2) even if you had mobs on you and enemy party came, you could regroup yourself and fight instead of just giving up the opportunity, as you got ported to wasteland and not in town (carebears hate it!)
3) HB grind for dynasty armors - so HB was only place to farm dynasty and especially from start before your party got geared with full S80/vesper with attribute, it was freaking hard so you actually had to go there and grind for looooooong time, people would have to pvp for spots because everyone needed dynasty set and thus everyone concentrated on one place, especially fights for Celtus which is something like hekaton, except that it wasnt "chance" to get something good, you always got something good but you needed many of those items for dynasty
Hellbound location back in the time, i would compare it to ToI in 2.0 saviors update, it was only spot where you could get tablets for 3rd class skills (which you needed) and you had to grind for loooong time to gather enough to get them, so people were fighting for ToI and farming it as much as possible, except the fact that in ToI you pretty much had no chance to farm if you were weaker side, but on HB there were several spots you could farm even if you were losing side (chimeras upper/lower spot, green spot, desert spot)
i think we are missing something like that right now in game, only thing which matters is xp and you can do that on quite many places and there are many ways to do it (and its kinda fast i would say), too bad tbh because people log for 20 minutes for SV bosses and after that instant log out, because why would you go GC