5Ds, for what it was blatantly based on, plus the fact that Dark Signers was well done and brought me back to the franchise. Writing perspective, gotta say original over the followers.
GX was by and large the worst of the series. The plot was slow and incoherent at times, the main is a sue whose problems rarely make any amount if sense, and it s overall a mix of a bunch of other series' elements mashed together and given cards. However, props to Sartorius for refusing to duel after he had what he needed... Well, until Neos Hax.
5Ds is card games with Spiral Power. Seriously, watch the series while picturing it as Tengen Toppa Gurren Lagann with cards. That's 5Ds. Yusei is a bad Yugi/Simon fusion.
ZeXal... Is marginally better than GX. That said... Its pretty awful. It takes elements from Megaman Starforce, Saint Seiya, and the like. Tron/Vetrix is like an Expy of Bermuda Von Vechtenstein from Katekyou Hitman Reborn, even his hatred fueled 69, And this list just goes on. Add to it, the main's annoying as all hell and butchers the idiot hero role to death.
Basically, you can't watch anything from this franchise without it ripping off/being an homage to/otherwise referencing at least 1 series heavily, if not more. They don't have any originality. Yes, it's true that originality is not everything, and that execution is superior, but... There's neither quality nor originality in the series after the original.