The best alternative system? Depends on what exactly you’re looking for. Pathfinder is probably the best kitchen sink, high fantasy, high crunch game on the market. That isn’t my general recommendation, tho, i personally recommend any storyteller system game or it’s adjacents, so World of Darkness (my personal fav, just not 5th edition), Chronicles of Darkness, and exalted. WoD and CoD are basically different takes on the same concept, our world but with magical stuff (vampires, fairies, mages, whatever you’re into) hiding in the background with very open character creation (no classes, mostly open and free form) and a focus on story telling over just combat. Exalted is wuxia fantasy with the same base system, but it does have a bit more focus on combat.

DnD does not do that, i think. Pathfinder has the focus on the character rather than the dice, some checks can be way out of range of a character that just doesn’t know something and some characters just cannot fail very basic tasks. In the story teller systems you just get more dice to roll as you get more skilled, which gives a good curve to how good your character feels at something while keeping randomness involved. Both of those systems really make improvement feel real in a way dnd 5e doesn’t.

Edit: and on my recommendations; the storyteller games are way simpler and cheaper than dnd.