After the release of d20 Future, d20M was one of my "first choice" systems for running my White Whale campaign: a crossover between the Planescape and Star•Drive settings run as two concurrent campaigns with players controlling a character in each.
In theory, it was a great idea for a system, and some of its spin-offs (Shadowforce Archer) were fun, but the whole thing was just broken after 10th level (well, 6th, but I’m trying to be generous). Also, handling high-tech equipment is hit or miss with the system.
It did make me realize that I like Alternity quite a bit better. Perhaps there should be a poll for that system?
Numenera. Love that system and setting!If we're polling diffferent systems, I'd love to see how Numinera performs.
I played it (several times) and liked it. It does suffer from the problems of 3era multiclassing (e.g., saves, attack bonus, etc.) that a 5e universal proficiency would do wonders for rectifying. I remember playing a gunslinger-psychic sniper in one game, and an investigator in another. I never got around to running one of my favorite mini-settings from Polyhedron: Iron Lords of Jupiter, which was a Barsoom/Flash Gordon-inspired planetary romance setting. I also liked a lot of the spin-offs that it inspired (e.g., Grim Tales, True20, Spycraft, etc.).
It feels clunky now, and I would probably look to other games first (e.g., Fate, Urban Modern Fantasy, Modern AGE, Cypher System, etc.), but I would still regard d20 Modern as one of the most important and influential books to come out of the d20 3Era.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.