GURPS was my go-to RPG when my group tired of D&D back in the early nineties and it's been a gift that keeps on giving. I ran a fairly epic fantasy campaign from 1996 to 2006 and have run and played in numerous smaller games.
Recently I've become enamored with the
Dungeon Fantasy Roleplaying Game (DFRPG) which some others have mentioned in this thread. It's a version of GURPS 4e that is focused on the dungeon fantasy genre. This means that you don't need to sift through all the material for other genres. The boxed set alone is enough for years of play. I've run two full campaigns and played in another one since purchasing it in 2017. (A friend is about to launch another one, too.) The role-playing club at my middle school uses it, as does the RPG summer camp that I run with a colleague.
It's got the detail of a good skill-based system, with some nods toward the niche protection of traditional class archetypes. Plenty of grit in the combat system, too. My favorite element of GURPS (and DFRPG) is the ability to build characters with a combination of advantages, disadvantages, skills, and quirks. It's easy to describe a wide variety of characters, and the system provides the mechanical crunch to support your role-playing choices. The mechanics of managing and fine-tuning character disadvantages is smooth and plays well at the table.
A final advantage, to my mind, is that you can pillage the enormous GURPS corpus for additional material if desired. Despite being a small fish in the modern RPG pond, SJGames still manages to publish a dozen or more GURPS supplements each year, plus some additional Kickstarter projects. While many of these are PDF-only, the DFRPG boxed set is a physical product. Gaming Ballistic also offers a great line of support material for DFRPG, focusing on the Viking-flavored Nordlond setting. A new hardcover bestiary will be published next month.