Of all of them, I'd have to go with the original series, although it had its dark points - namely, everything after Yami Marik was beaten but before the last season, and the first series in which rules didn't exist and it was more like Bakugan than Yu-Gi-Oh! - at times. A fairly good cast, a great finale, and some very exciting duels, even if the outcome was obvious and they dragged on unnecessarily at times.
GX...I didn't really care for. I just couldn't like Judai as a character and, whilst the second season was much better than the first - see Judai getting trashed by Edo Phoenix was probably the highlight for me - it wasn't good enough to get me into it...and then there was the third and fourth seasons, which pretty much shut my interest in the franchise down until 5Ds started airing.
I really enjoyed the first season of 5Ds, but from that point afterwards I hate it even more than I do GX, because they absolutely destroyed Aki's character, and I will never forgive them for that. The series finale was worse than Death Note's, and that is shocking, considering how promising it was when it first aired. I wasn't overly bothered by the "card games on motorcycles" thing, to be honest; it was ridiculous, but it was quite interesting as well.
Zexal I've not seen...I'm reluctant to even give it a try, considering what a few of my friends have told me, and they know my tastes fairly well. If it ever gets shown on TV here I might watch an episode of two to see what it's like, but nothing I've seen really makes me want to watch it.