I like level-less spells. I find using them and allowing them to scale in power by the discretion of the Referee and player to be a really elegant way of having magic in OSR games. For my ruleset I use the 1d100 level-less spell list from Knave by Ben Milton.
I like to use the Knave set of spells because they're also very utility focused. They allow the caster to do interesting things, but mostly to overcome situations and manipulate environments rather than do damage in combat. I like for fighter type classes in my games to be the class that is really good at combat, the specalist/thief type to be okay in combat but better at skills, and the magic-user to be really weak but good at random problem solving.
However, I do find that using a level-less spell list like this of simple spells tends to make magic seem a little bland or pedestrian. At first I was going to use some kind of spell name generator to theme a characters spells but couldn't find a good one.
However, I came across the idea of using grimoires in the Hark A Wizard supplement by Zzarchov Kowolski. In it he details that it made his game more interesting because grimoires containing spells tend to be evocative and give something for the Wizard players to track down to learn new spells. If you'v ever played Call of Cthulhu, which he cites as an inspiration, you know what this is like. He listed a bunch of samples:
Titles: Proverbs of the Unknown Philosopher, The Eternal Mountain, The Quivering Reed
Description: This simple chapbook is composed of reed paper pages in-between sandalwood covers bound together by small leather hoops. Written delicately within using a stylus and black squid ink are the 36 proverbs of the Unknown Philosopher. These simple kaons have driven many a hermit to madness and are said unlock the innermost door.
Language: Japanese, Sanskrit
Occult Truth: All beings (including player characters) reincarnate up or down the great chain of being upon death. It is possible to trigger memories of other lives.
Champion: a champion from another dimension is summoned. It is able to answer most questions, and may aid you if it finds you worthy.