"Leveling" isn't the same as getting leveled. That's an important thing to notice.
If I'm taking my time, I'll run Tristram for gift boxes until I hit lvl 18 for the new skills you get at that level, then I'll rush through the rest of the act and hit about 22-23 after Andariel. Sewers can take you up to 32ish with a couple random xp potions you might find, butb I generally stop at 30, then rush through the act and get a couple levels through Arcane and Canyon/Tombs til about 34, rush act 3 til trav/durance (the que-hagen mobs in durance levels especially give a ton of xp). I've gone up to lvl 46 just farming Mephisto for Arcane Crystals to upgrade gear and prepare for Level Challenge 1 at lvl 48 or 50 (48 cause you can put points into skills again every 6 levels), but often will do it right at 50 so when you do the cold priestess, you get +2 skills right away. From there, I'll go through act 4 and act 5 pretty quickly and hit hatred cows at about 54. That will take you to 60 after 2-3 runs usually, at which point you stop getting exp per kill, mostly. Then I'll go to hatred Island of the Sunless Sea (if you have a character that can do well enough and survive well enough in there, you can farm K3KBA for flawless emeralds to cube into perfect emeralds, and MO crafting points on rings to get 500 CP, which lets you make greater xp potions). With the greater xp potions and decent kill speed, I usually hit lvl 72 after 2 runs there. Then I'll go on to Terror.
That's generally my process for hatred.