There's some cool tower defense games with big skilltrees if you're into that. Dungeon Warfare 2 is my goto.
ToME 2.3 had my favourite implementation of a skill tree- it did synergy bonuses in a cool way where you could exceed your level cap in a skill by investing in a main skill and then a subskill, like getting combat to 20 then raising swords to 20 might boost combat to 26. This was useful because hitting 25 combat might unlock cleaving hits as a perk. Made for a lot of interesting decisions about how to build, whether you wanted more power right now or in the future, etc. ToME 3.0 did something even cooler where you could find teachers that would increase efficiency of skillpoints spent in the tree, sadly it was only really used for a goofy DBZ roguelike and was never finished. It's a shame, it had lots of cool ideas like going around crushing dojos, gathering the dragonballs for wishes, and a linked afterlife and karma system that was also tied to your spirit skills as opposed to martial arts.
I'm looking for games that skill tree decisions matter.
PoE is a good one on that front, but much like Diablo 2, there's not a lot of decisions to make. Most of the tree is either mandatory or filler. I'd say GD is better, at least the constellations part is cool and interesting in how it's interconnected and the differences between them are major.
Salt and Sanctuary's tree is garbage. Like 90% of the nodes are the same shit, +1 to a base stat. You just path to whatever weapon you want to use to unlock it and grab nearby flask charge nodes and relevant stats.
Last Epoch has good skill tree stuff- you've got a skill tree for your generic class, another for your specialization, and then a tree for each skill you use. The first two use different points than the skills. The result is a very customizable character and a ton of build variety. Sadly a lot of the skills are kind of boring, but it still has some pretty cool stuff. Druid and Necromancer both do some really weird shit, and Paladin has a time looping skill I really like, though it got nerfed (deservedly) pretty bad I think.