I played through all of them entirely except for Hextech Mayhem, and got all the achievements in Mageseeker.

I’d say in terms of the game with the most content and the most fulfilling that probably goes to Ruined King. If you enjoy the turn based combat then you’ll likely enjoy everything about it, the story is pretty good but what makes it are the interactions between the characters, you get some cool progression by having each character find their legendary weapon, and you get rewarded for figuring out which team comp you want to run. The only gripe I had with Ruined King when I played it is the final boss is a huge spike in difficulty relative to how easily you can blow past most enemies by the time you get to him. It was a bit jarring.

Mageseeker’s story is about on par with Ruined King, but the amount of fun interactions (and voice acting) is fairly minimal so you miss out on that. The action combat is really fun and they translated Sylas’s kit to the game pretty seamlessly so if you’re looking for the league gameplay experience this may be the best one for you. It isn’t particularly long but I played on the highest difficulty and was able to get some milage out of that.

CONV/ERGENCE loses out in the story department. The progression of what you can do in combat, the combat itself, etc. are very fulfilling and you get to do some whacky stuff with the timewinder and all your time abilities but you really miss out on a deep interesting story especially if you care about the lore at all, because lore-wise this game quickly makes no sense. The only interaction I particularly loved was between Camille and Ekko. It is certainly not a bad game, the combat is great, environments interesting, fairly challenging, it just isn’t as a complete package. Again if you’re looking for a league experience gameplay wise, this isn’t too bad either.

Song of Nunu, and I mean this in the best way possible, is a glorified walking simulator. I do not mean this to disparage it because it is actually really really great! I love the Freljord and the environments are just beautiful, but this isn’t a game about the combat or the mechanics, its a fairly short puzzle game where you walk in a mostly pre-defined path to get to your destination. My biggest issue with this game were the keyboard controls but playing it with a controller might alleviate that, I just didn’t try. The story was both short and compelling from the eyes of Nunu. I don’t know that it inherently worth the price tag but I don’t think you’ll be disappointed paying full price, it was really enjoyable.

I have yet to really be disappointed or dislike any of the Forge games, I think they each have some flaws (probably especially length vs price) but I have never outright said I wasted my money, they’ve all been pretty fun.