To me it's The Longest Journey, hands down. First of all, it's a point-and-click game, and the whole reason why I discovered the games was because I was asking around for recommendations in that particular genre. Second of all, the voice acting and the script is brilliant, I love every single character and pretty much every single aspect and moments of the game. The story is great but also woven through with mystery, there are so many locations, April is an amazing character, and we get immediately immersed in her journey. Third and most importantly though: the world building is just fantastic, and the excitement that the discovery of the twin worlds and their mythology brings is something that by default none of the follow-up games could ever achieve.
Dreamfall in comparison is different, and I admit, it left me quite frustrated the first time I played through. Since then, I learned to appreciate it, but it's still my least favorite of the series so far. Some of those are personal preferences, I wasn't a fan of the fact that we had stealth and combat in the first place, and it made me hate locations like the caves and the hotel. I also felt like some of the voice acting wasn't up to the same level, but that may just be nit-picking. But mostly, I was frustrated by how the game ended, with no sense of closure and a whole bunch of open-ended plotlines. Yes, I know that Dreamfall was never meant to be a standalone piece, but that shouldn't have meant a total lack of resolution. And as much as I loved how dynamic the game was, the fact that most of the puzzles can be solved with your phone was really disheartening. That all being said, Zoe was a protagonist that I actually really needed at that time of my life, so it is still a really special game to me, but it just can't beat TLJ for me.
Dreamfall Chapters so far feels like it will fall between the two. The puzzles are more creative than in DF, I'm more happy with the voice acting, adding Choice and Consequences is a great fit for the franchise, we also seem to be getting back to some of our old mythology with the House of All Worlds... Overall, I would say that it's actually pretty close to TLJ so far in my book, although I wish we had more locations to play around. (That could of course still change by Book 5, and it seems pretty likely that it will.)