There are probably better ways, but a decent system I've worked out is...
Start Tristram runs as early as you can until about level 15.
Then do Tomb runs from 15-25.
Next do Chaosbaal runs until 40.
Follow it up with nightmare difficulty Chaosbaal runs until 60.
After 60 just go to Baal runs until around 70.
Finally, do hell difficulty Chaosbaal runs until Chaos Sanctuary stops giving enough XP, and then like in nightmare, switch to doing purely Baal runs. Unless you're really dedicated you'll probably want to stop around 90-95 because of the diminishing returns on experience at that point.
What about getting power leveled by someone who is 80+? Based on the experience sharing explanation it sounds like the high level character would take the largest cut of the share, right?
Also people seem to be into bringing their maxed out Chant Sorceresses to games. Since the buff can be cast on any level character, players get insane amounts of +fire damage to their attacks. Doesn't matter if you're a caster, better to swing that being the case.
Alright I slapped together a small guide for power leveling based on info here. I'm not an expert on this so if anyone wants to contribute feel free to update the page or reply in this thread and I can add the changes.
I think it's important to reflect that Chaosbaal runs aren't about clearing the Chaos Sanctuary or the Throne of Destruction, but rather doing both so as not to do too many games too quickly & get temp banned.
Looks good otherwise. Thanks for putting the guide together.