Just a list of my favorites to link, in no particular order :)
The Game Dev Tree
Just an awesome mod with unique mechanics! Not much else to say :)
ArcTree
Not popular enough! Great mod that manages amazing ambience without any sound xD A bit grindy in the beginning and occasionally you might feel a bit lost, but definitely worth it
The Comunitree
Just… Wow. Very slow at some points, but VERY fun. Highly recommend. I would go into the mechanics but I don’t want to ruin the surprise ;D
Prestreeestuck
Wow! I hadn’t played this for a while, only trying the start and not progressing very far, but I’m starting again (and I may or may not have turned on devSpeed), and it’s amazing. Like a few other trees on this lists (arctree, the camellia tree), I don’t get the lore in this but it is still fantastic. The layers, though I haven’t unlocked very many (Almost done with doom challenges, act 0, for those who know the game), are very unique and I love how they’re arranged in a circle xD
Falling Mountain’s AlterPrestige
Not much to say but very fun! Very little grinding (at least as far as I’ve gotten) and interesting mechanics! It also, as far as I’ve seen, doesn’t have any direct “boost points by points” upgrade, making it very easy to know “I’ll probably get this many points by this time” as opposed to other games where the gain increases with the points and it’s hard to know exactly when the production will die off.
The Camellia Tree
This one, unlike many of the others, is still pretty new, but it doesn’t make it any less fun! Very good game! (Even if there is some lore that I don’t get lol)