I typically find Tristram a good place to lvl until around 12 or so, and thats when I usually start Catacomb or Sewers Runs. Arcane and Canyon of the Magi are good places to get to lvl 20, and once at lvl 20, Normal Cows is the way to go. Nightmare cow exp starts only at lvl 25 and is a good way to get to around lvl 50, and after that, you can nm cs to lvl 60.
In pubbies: Do act1 and 2, try to get lvl 20. Lvls 18-20 are the hardest. Join any game with a big party that can help you get lvl 20. After lvl 20 join cow games till ~30, go straight to nm cow games till ~60, put all your best gear on and go to hell. In hell some parties work some dont. Some chars can do everything on their own some need help vs imunes.
For my new chars, I usually have one of my friends go on our shared acct chanter and enchant me (using a bow until lvl 15 and then switching for Ravenclaw uni Long bow). By having a chanter, it takes roughly 30 minutes to get to lvl 25, and with the help of my other friends foher for conviction, you can lvl from 25-50 in about 45 minutes in NM cows.
the best way to lvl in classic is go to a pub game and lvl to about 10 doing some quests. Then do trist runs to about 13. Get a friend to andy run you to about 15. And 15-20 do tomb runs with a friend. 20-26 norm cows. and from about 26-55 have a friend raid nm cs with you. then hell cs
Get to wear raven claw bow and get a friend to enchant uyou and solo tomb run. Gauranteed lvl 14-30 in less than an hour (expecially if you find a xp shrine and cow with enchant). After that, join in on pub flayers in hell and watch the levels fly. After lvl 75+ or even 80 start doing hell cs.