@Stilgar: Act 3? You like the Jungle areas?

I thought everybody hated them!
Items:
Mephisto is of course the easiest item run and he drops very well, but remember he cannot drop many of the best items in the game such as Windforce or Grandfather or Tal Rasha's armour. Also I think Meph runs are quite boring, I ususally do a few each time I play but that's it.
Baal is also a good dropper, but takes a bit longer to get to. Pindleskin is of course a favorite item run for many - if you don't kill Nihl, the red portal will always be there and thus you have immediate access to Pindle, who is also a very easy monster even in Hell. One static and one meteor and that's it, if you are a Sorc for instance. Pindle can drop every item it the game.
For XP, everybody knows the Cows and Bloody Foothills...Halls of Pain, and and the Ice caves in Chapter 5 are also very good for XP. But it depends very much on what lvl you are, after lvl 85 or so it is certainly only worth playing Chapter 5 for XP. Tal Rasha's tombs are good as well if you are lower level.
For Scenery: Arreat plateu where the Ancients are. Before you click on the book, take a walk around the area and admire the view!

As soon as the Ancients have come alive, there is no time to admire the view
Personally, I prefer doing combined XP-MF runs rather than doing 100 Meph or Baal or BF runs...every Superunique has good dropping possibilities in Chapter 5, so a typical session could be like:
Hit Meph first, then go to Chap 5 and hit Frozen Tundra Waypoint (WP). Take out the Superunique Eldrich, do a BF run and you get Shenk and Dac farren (both Superuniques who can drop fairly well, I think the hubby got Tal Rasha's armour from Dac) on the way. You can as well look up Tresh Socket at the Frozen Tundra, since he can also drop well. Then go to Glacial Trail and take Icy Cellar and Frozen river. You'll get a lot of XP and several Superuniqies who drop well. Hit the Halls of Pain and run around for XP, then go to the WP in Worldstone Keep lvl 2 and do a Baal run.
This is of course if you have finished the game. For lower level charaters, there are many good areas depending on charater level. Tal rasha's tombs, Arcane Sanctuary, all the Superuniques in Chapter 1...(Chapter 1 and the Tombs are dangerous though since many monsters there spawn MSLE at Hell).