In general any old world zones are out of the question if you're only concerned with grouping, exp and loot, that is with the exception of those which are current hotzones. The only old world zones which I continue to visit are Unrest from level 10-15 and Lguk 30-45. The reason I like these is fairly simple.
Unrest: The yard trash doesn't run and there are so many things to kill its non-stop action.
LGuk: When you hit 30 those undead frogs give pretty good exp/cash/FS weaps. Since you'll be with your friend hopefully one of you will have a tank and the other some healing class. It should be a lot of fun hunting the bed room and the spider cave.
Many people here could give you other/better suggestions but I kinda partial to those zones. Other old world zones I like are oasis (orc highway) 15-20 and solb 45+. From Velious, Lake of Ill Omen was great when RoK came out but with the ZEM its not worth it anymore and Crystal Caverns/EW (Rygorr Camp) in the mid 30's to mid 40's was great also. I left shortly after that so dont really know much about any other zones. By the time I came back most people just did Ldons until they could start going to planes.
My current path with my war/clr combo has been/will be:
1-11 tutorial
11-15 Unrest: Yard Trash/Round Room
15-25 Paludal Caverns
25-30 Unrest: 2nd Floor
30-45 Tower of the Frozen Shadow(hot zone)
45-50 LGuk (Safe Hall)
50-55 Howling Stones
55-62 PoV
and so on...
Anywho, this is probably more info than you wanted but I hope it makes your decision a bit easier. For me it was more of an impulse to come back.
Dartanicusx
63 Paladin
Povar