As a person who watched most of it...
...The first and last seasons honestly are the best. The first season just fucking throws the rules out of the window so it seems less like a card game and more like some sort of fight with actual summoned monsters. It's joked about a lot by fans, but honestly I think if the anime (and its spinoffs) continued like that it'd be far more interesting. It may have a bit of a lull, though, but I had fond memories of it.
Season Five legitimately is awesome because there are so little card games. In this season, they finally fucking get past the filler and now go back to a memory of Ancient Egypt where the Pharoah himself once lived, and honestly it's pretty damn cool; it shows origins of some of the most iconic cards in the series and whatnot. Also Thief King Bakura's laugh.
The seasons between honestly weren't as good. ADMITTEDLY, I never completely watched the Orichalcos arc, and it might have had some cool moments.
I've told this to people before; the main reason people think it's a kiddy anime is because 4Kids tried to change the presentation of a lot of the story, everything from changing names to outright removing certain parts (the most infamous is the whole removal of guns, making it look like guards were using their fingers as guns), but the Japanese version actually was pretty good.
...There's also another season, that was not dubbed. It was deemed "Season 0" and was based more closely off the manga of Yugioh. Season 0 was pretty damn good in my opinion, because it was about all games, not just card games. Also the fact that Pharoah was... well... he was pretty damn awesome, especially in the beginning. I'd recommend S0 to anyone who wanted to watch yugioh but didn't care much for the card games, as it does introduce pretty much all of the important characters and it has some cool moments in between.
Take this with a grain of salt as I'm a rather biased source.
tl;dr Season 0, Season 1, Season 5, subbed. Spinoffs-wise? Hell if I know, but I'm going to be staying with Arc-V if any of you want to know how that turns out.