If you think GX is more lighthearted.... prepare yourself for season 3. You should definitely watch that subbed though, since the dub demeans the tension and stakes of it. The first two seasons had that as well but not nearly as bad.
As for Yu-Gi-Oh as a whole.... I loved the first series. It was a huge part of my childhood and I owe it for that. I still have my massive collection of cards because of it, and am still collecting. xD Sure, the dub was bad but I can still watch it today and like it because of the nostalgia value, it was just part of the package. I'd also point out it was a hell of a lot better than GX's dub, which threw in phrases like "get your game on" to "relate" to the hip new kids. The original YGO dub could have some real serious dialogue (even if it was about children's card games), and didn't cram in as many puns (though they were still there) as they could in every line of the script. Nor did they give characters random accents or character quirks like GX did, I remember characters all over the place having accents for no reason and off the top of my head some Quirky Miniboss had a thing where he would rap everything he said, again, for no reason at all. Meanwhile the first YGO had every character cast with a voice that fit them without cheesy accents or added on quirks. Don't get the impression I have something against GX because I don't, I liked it a lot, I just thought the dub was godawful. Not saying the original dub was perfect either, because they censored a lot of things that were integral to the anime (lol Shadow Realm).
And yeah, even as a dumb kid I knew they weren't following the rules at all in season 1. But having read the original YGO manga recently, I know the reason behind that. Believe it or not, the manga that started this all wasn't about Duel Monsters at first. It was about games in general, thus "King of Games". Kazuki Takahashi would have the villain of the week do something terrible to Yugi and/or his friends (actually terrible, not something about children's card games), then Yami would come out, challenge them to a Shadow Game, and punish them with his magic powahz in a karmic way once he won. Takahashi would also introduce a bunch of games he made up (Capsule Monsters and Dungeon Dice Monsters being two of them) and have Yugi play them against the villains, Duel Monsters was originally just another one of these games. And guess who was the villain of the week for the Duel Monsters centered chapters? Seto Kaiba. When the author decided to bring back Kaiba and the game for another arc, where Kaiba would serve as the manga's first real arc villain, his popularity, and by extension Duel Monsters, skyrocketed. After returning to the old formula for a couple more chapters, Takahashi's editors convinced him to make an long-term arc about Duel Monsters since it was being received so well. Thus we got the Duelist Kingdom arc. The reason the duels in this season didn't adhere to the rules of the game was.... well, there weren't rules yet. Takahashi created Duel Monsters and set the foundation with the most basic rules, but that was it. He wanted the duels to be more "exciting" I suppose and more like a real battle between monsters rather than a game following strict guidelines for gameplay. By then, the official card game in real life was being printed by Bandai, who kept the game simple like the manga was. It wasn't until later was the game bought by Konami, who built on the game's rules to create the Duel Monsters we all know and play today. So the Battle City arc, where the series started following the rules now, was Takahashi adapting to the real life equivalent of the game so his readers that were collecting the cards and playing them by the Konami rules wouldn't get confused. That also explains any inconsistencies between manga/anime versions of cards and their real life counterparts. Takahashi was the one who made the cards in the first place, and the anime only followed his example to make a (for the most part) faithful adaptation. Konami however, were the ones who decided to change the card types and/or effects of cards for the real life Duel Monsters. In the end, all that early installment weirdness of the game mechanics in Duelist Kingdom wasn't Kazuki Takahashi's fault, or anyone's really. It was just one big mix up.
ANYWAYS. First series? Classic. I'd highly suggest reading the manga though, since it's a lot better (especially the first 60-ish chapters where it isn't focused on Duel Monsters), and it's a lot less ridiculous on children's card games being the center of the universe. In the anime for example, Dungeon Dice Monsters was treated as something that MIGHT be as good as Duel Monsters. In the manga, it was just another game Yugi and his friends played, same as Duel Monsters. Still love the anime in spite of that, which gave us some pretty cool filler arcs. I still need to get around to watching season 0 since it's based on the chapters before Duelist Kingdom.
GX, I also liked a lot, and was the height of my card collecting career. I have more GX generation cards than anything else, I have so many Elemental Hero cards I don't know what to do with them. While it does contradict the Egyptian background for Duel Monsters established in the original premise of the series, the supernatural elements were all well-integrated and overall had a cool plot with a nice cast of characters, even if they weren't as appealing as their predecessors. Most of the supernatural aspects can be attributed to the series being an affectionate parody of the old YGO having children's card games being Serious Business. Something I could appreciate.
5D's is.... where I cut off from Yu-Gi-Oh completely. Say it with me folks: CARDGAMES ON MOTORCYCLES. I... couldn't do it. I still can't. I've heard the anime has a good story and cast if you take it for what it is... but I can't get past that part. Not only that, this is the series that introduced Synchro cards. I hate change, and that was just one too big for my card game playing self to take. I don't know how they work, I don't care how they work, I don't want to know how they work.
I'll give 5D's this at least: it's not Zexal. That anime looks so ****ing terrible. The kiddie animation style, character designs, and tones. I don't care how good the rest of the series is, I will not be able to tolerate it as long as Yuma Tsukuno is the protagonist. Even the most hardcore fans of Zexal admit he's intolerable and the worst thing about the anime. And they're further raping the card game with these Xyz monster things, which make Synchro cards not look so bad by comparison.
I'll forever love and be thankful to Yu-Gi-Oh, even if I haven't liked the direction it's been taking the past few years, and no matter how bad it may get in the future due to the people in charge wanting to milk it for everything it's worth.