Preferably I'd like as little change to happen on level up without my consent as it can. That means: No bonus hp/damage/chance to hit bullshit, and that goes for enemies too.
Arcanum and the early Fallout games did leveling the best imo, New Vegas with Jsawyer.esp followed shortly after. Fallout 3's was definitely the worst in the series, and awful in general, Perks to raise things that get easily maxed, tons of useless investments, the ability to raise a special 10 times, then another one to raise them all to 9 anyway. Too many skills, perk every level, etc.
All in all though, the absolute worst levelling system I've ever experienced was Oblivion, anyone who defends the meta gamey feel of that terribad system hates RPGs and fun. Awful, terrible, idiotic. You can run around in circles or jump in place while casting a spell to level, and it isn't dynamic at all, so the grind is pretty much encouraged. That's not even the worst of it though, the level scaling, the game is actually easier if you never level. Then the moronic levelling system, which so easily could have been good. (Make all skills count towards leveling a la skyrim, and then offer the stat increments based off of what you used same as it did, sans the meta game shit needed to ideally keep up with the stupid level scaled world. Would have been good for what it is.)
When it comes to new titles, Shadowrun's was okay, Pillars of eternity had potential but was lacking to me, not because of the leveling itself, which didn't force much on you and also gave options, but mostly because the options weren't interesting to me. I like an RPG system that makes it hard to choose based on the sheer number of attractive choices, where you can't wait to level to pick on of the perks/skills/abilities. PoE was not that game for me, i had more trouble trying to work up the will to choose which insignificant increment was the least useless. Divinity Original: Sin on the other hand, was that game. If it weren't for the awful chance to hit scaling, mid game respec, and retarded late game hp bloat, it'd be one of my favorite. Unlike PoE there was always something cooler that I wished I could get, and at the end only just enough points to go around to get some. Which would have made for unique playthroughs with good non-story c&c, but being able to respec before any fight killed it.