assuming you aren’t getting rushed, and you’re just playing through all the quests on a new character, i mean.
for me it’s the 50’s. you no longer get any good xp from normal mode baal runs, and normal mode doesn’t really drop any gear you can use unless you’re planning on upping some specific uniques. on top of that, it’s a real slog to quest out act 3/4 at those levels unless you’re using one of the cookie cutter builds
Below level 17, my character is soooo slooow because I can’t wear Stealth yet. 
15-25 because finding tomb games is nearly impossible with this nonsense games list design.
Depend on which class … On a sorc, below 24 and 27 since you can’t have spirit and insight.
On a melee ? Anything above mid nightmare until i find a weapon that allow me to clear everything.
1-30 on a Frozen Orb Sorc (playing through with build in mind not respeccing) is painful to me. Icebolt’s damage is weak, you use no other skills between it and Frozen Orb to upgrade to, and mana pots go chug chug until you get Insight.
Overall, 20-24 is a slog online after the initial reset rush and you are trying to catch an Ancient’s carry at 24 to get to Baals at 25, unless you have twinking gear for leveling. You rarely see Cow Games hosted in Normal for helping people level, and Tomb games are harder to find, as Bodach mentioned.
the solution i’ve found for tomb runs is to start my own tristram runs once i hit fifteen. keep running them until you get a full group following you, and let them all know you’re going until they’re all 15, then you’re gonna blitz andy and act 2 all the way to tombs. with the players split between ladder and non-ladder, there’s a dearth of “-runs” games of all types, on all difficulties, so that changes my metric for the experience gains.
yeah, it might be better experience to move on, but realistically it’s faster in the long run to just keep running tristram, or tombs, for a lot more levels. i do tombs all the way to lvl 30 so act 3 is easier, chaos to level 40 so i can blitz act 5, and then baal runs to level 50+ before starting nightmare
levels 20 - 24. If I’m playing a caster character with decent AOE (Sorc, Elemental Druid, Necro, Trapper) I can usually get to ancients by level 20 and need to back track to gain levels. It feels like nothing in act 3 or 4 or 5 gives experience despite me being able to kill them in droves 
Seriously, why can I get more experience from tombs in act 2 than from anything in act 3. It is dumb. Even if XP gain is not improved on later levels, I would love it if it were improved on Normal difficulty.
If my character can kill it, it should provide decent experience. If I am under-leveled for it, all the more experience I ought to be awarded (when not leeching from higher level characters, of course).
it honestly depends on the build and the class… and even your luck.
a single element sorc is going to hit a wall as soon as they hit hell.
a melee char may hit walls at any point if they cant find a weapon upgrade.
that one right there hits me in the feels. the roughest one there is assassin and 3 socket claws. no 3 socket claws drop before hell, and even then it’s mostly 1s and 2s, so you gotta waste a larzuk on it if you want to use malice or pattern while levelling an assassin
I never feel like a sorc hits a wall because she can just teleport past or away from immune monsters.
Attack rating characters just feel bad and slow the whole way through because the chance to hit is so bad when under leveled, and still not great in hell because of monsters’ block chance (maybe that is just bosses?)
Not finding a good weapon upgrade? Shoot… I never even consider playing an attack rating based character until after finding the usable weapons with a caster character first.
lvl 27+ without insight/spirit
lvl 1-24 because it takes hours if you go through the quests and hours if you try to find exp games lol…they nerf’d the exp gained from high lvls running lower lvls through games for exp, it’s really irritating.
I think the nerf is many years old, but I never liked it anyway. having a lvl 90 run trists, cows, tombs, norm baals…they got nothin better to do so why not? right? but then it got nerf’d out.
so all the low lvl exp runs are almost non-existent. once you hit 24 you can do ancients, and then baal runs. from then on it’s semi-smooth.
but then at lvl 80 you see exp drop by a lot, and by 85 you cant even get 1 lvl from a baal run anymore. by lvl 94 you need 100’s of baal runs for a single lvl
so…the lvl range I dislike most is 80+ because of how impossibly hard it gets to level up. 50 baal runs is like 1% of a lvl by time you hit 95+
5000 baal games for a lvl is downright absurd!
20-30. Because your gear and skills suck especially Barbs. Once you get your level 30 skills then it’s all uphill from there.
40-60 for me. the skill point doesn’t make hybrid build viable in hell difficulty. and a solid cold build can’t do jack on the monster that has cold immunity. the merc is also terrible without decent gears.
I am a noob so this experience is personal, you may not be in the same situation as I did if you know more about the game than I.
hell’s immunities are the wall every player hits, old and new. especially for the start of ladder when you don’t have stash tabs full of good gear and you’re still muddling your way with that first character so you can start farming for the good stuff
The trick is to not do Hell. At least how you would do normal or NM. Just run through or teleport to the finish the important parts and then do CS and Baal runs. Hell is impossible to solo without super good gears.
but you see, leaving the quest incomplete bugs me 
it’s like I brought it up to myself to be suffered. I think I may have OCD on a certain level.
either being just below norm ancients (22-23) or being at my level goal of 90 (88-89)
95+ Is mind numbing.
I agree wit this in every way