1-13 Tristram runs
13-24 Tombruns, can do canyon of the magi too. Typicall takes the longest time, but keep at it, be diligent, just spend those two hours grinding away, and try to find multiplayer games to increase exp. And skill the character right. Don't get too hung up in picking up lowgems, runes and items, it takes too long.
You can actually also do travincal runs from level 21. You get some kind of decent exp there. But only if you're certain you can grind them properly and efficiently. They can also drop decent items.
24: Get rushed to ancients normal, or just start going there from tombs from about level 20, you'll be about 24 then when you get there. You get one whole level of exp here, so being at 24 exactly is very helpful, it is a tiresome level to grind that last one.
25: Normal baalruns. But only if there is baalruns going online. There usually are about 3 weeks into a ladder.
26: BUT!! You can also actually do eldricht n shenkruns. You have to do be level 26 then, because the baal's minions are level 31, and only 5 levels lower do you start getting decent experience from monsters... lower than that, you only get 5% which is killingly low. It is easy runs, they can drop decent stuff, and it's much easier than doing baalruns, which can be very hard if you're not geared properly.
40: Normal baal - get rushed, or go to nightmare. Get to nightmare ancients. Get them done yourself or by others. Do nightmare baalruns until level about 65, but you can actually do them until level 77 or so. You get decent exp.
Level60 +++ : Hell ancients ready to be done.
D2:R