For xp bonus zones, the Hole or Scarlet Desert get my vote.
Both have challenges in terms of getting there (shrink/invis for Hole, transport to SD - which is solved via GH port to twilight sea). I'd recommend gate potions in either case to get back, or otherwise face a tough time getting back.
Scarlet Desert Grab a gate potion, and port via GH (for about 65pp) or wiz TL to twilight sea. Zone is almost straight west from port in area. Invis and run up to the mesa top. This is generally a very low risk place to xp due to zero add factor, and easily split/pulled mobs.
Watch out if the ESR 1.5 epic mob is up in the little alien temple, otherwise, the tro jeg fire servants and captor are easily single pullable. The warders are casters, and iirc I didn't mess with them because of their dispels.
On the other side, the revenants (straight melee w/dot proc) and reavers (SKs) are also single-pullable. Unless someone else is there PLing or quadding, there ought to be enough for you to fight non-stop (if you can) and earn XP up to around 60 or 61 when they start to go light blue. If a caster is quadding, they may let you clear the Reavers since they can't be quadded easily. All the named (Magistrate, etc..) are revenants and straight melee types.
Any area on the mesa top is safe to AFK, there are no roamers up here. Again, the Elite Sun Rev 1.5 epic mob is the only thing to watch for. If he's up, you can still pull reavers/revenants from the ruins and just avoid the tro jeg temple and ESR. If someone is raiding the ESR, I would definitely advise leaving; he's got a nasty AE that can take ou out.
Some info on the bread-and-butter mob up there:
sun revenant Loot is bad, with just a few items sell for 50pp in baz, and vendor trash that sells for 1pp or so.
The Hole If you invis up in the Hole, you can run down to the see invis line (past the elementals, near the NPC named Cogswin who spawns by the little pond). Two constructs will roam up to this point where you can single them, but watch your back, as elementals will also roam down to the end of the hall and may add if you're too close.
From that point the constructs from the room below can be single pulled pretty easily, though watch out since several of them will wander in rotation. They start to go light blue in the upper 50s though.
Just below that is the entrance to the old city, with plenty of ghosts that are single pullable at your level, as well as gargs on top of the wall if you want (gargs will add if you get too close to the wall). This area with the three static ghost spawns is safe to AFK. Oddly, the ghosts DO run at low health, so snare and lay a beat down. One other heads-up, a construct from the room above will roam down to top of ramp, so if you are on the platform too close to ramp, he may add.
Past that entrance, there are two roaming constructs that might add as you pull ghosts, but otherwise, the buildings are loaded w/ghosts to pull. As I recall, this area definitely goes light blue by upper 50s as well.
Past that, the mobs don't really change lvls until you get to the tunnel, which can be difficult to manage due to quantity of adds.
Loot here is pretty good, with tradeskill items (mercury, sulfur, oxides), and stuff like gems, earrings, bracelets and other vendor sellable junk.
Edit note: added comments regarding loot. Edited, Tue Dec 13 10:23:53 2005 by traderelf