Season 0: I love it for Cute Yugi and Yami Yugi 3nodding Also the other characters are more developed than in DM
DM: Was my favorite for half a decade, but it lacked of character development and I only cared about Yami Yugi there so ... (and why not, he was the main plot, and it took exactly 224 episodes for him to find his answer neutral )
GX: Im starting to watch it, I think its ok for now.
5D's: My favorite one I totally love it, screw that people think about card games on motorcycles as dumb, Synchro Summons and Riding duels are awesome, the characters aswell except Akidon't judge me, I just don't like her, and the story 3nodding
Zexal: I watch it once in a while when episodes look "interesting" although these newer episodes are better than what we got at first. I don't like the characters so much, and the colorful enviorment is making me go away from the show more than the characters already do xD
5D's is my absolute favorite. I love the characters and the setting and the ~card games on motorbikes~ and the Synchros and...everything. Seasons 1 & 2 will always be my favorites (the Dark Signers were just so awesome) but I pretty much love all of it.
Zexal comes in second. I actually only caught up on it recently (I didn't really like it when it first came out, mostly because it was so different from 5D's) and I didn't expect to love it so much but...omg. It's amazing. I still don't like the sillier episodes quite as much, but when there's plot oh my god is there plot.
GX is third, mostly because I haven't watched all of it whoops. I've pretty much only seen season 3 & 4, which were awesome...but overall there's fewer characters I really like and the setting isn't my favorite.
DM sadly comes in last because of pacing issues. And also there's only a handful of characters I really loved. I still love it! It's pretty much impossible for me to hate any YGO series. But I don't love it as much.