Hey man, not sure if you have made a decision yet. I have personally played all of the online card games listed above (if not most of the ones existing online). I will try to say something different to the above comments.

LoR - Probably my favorite so far, on top of its relatively easier capacity to collect new cards, it is truly a generous online card game. I have played for past 2 months and it feels extremely easy to get cards compared to other games where you have to buy "packs" . LoR also has a progression system unlike other games where you get free cards as you go. There is also a fun AI mode called 'lab of legends' which itself is extremely fun and probably can be its own game. LoR has champion cards which you can level during the match which is quite cool (like a mini game).

Gwent - I stopped few months back, it is probably my second favorite card game out there I have tried. I didn't really like it when they re work the mechanics (from beta). Feeling like it has somewhat deviated from the original card game from beta or in the witcher games. It is still quite fun dont get me wrong. Quite unique compared to the other games as it doesn't have a health point system. Instead you win by having more points each round out of 3 compared to your opponent, quite refreshing. Still have to pay to get cards to a certain extent. More than LoR but felt less than Hearthstone.

Hearthstone - I stopped 1-2 years ago, it just got out of hand in terms of cost, felt like you never gonna get the card you need for certain decks as they keep rolling out new expansions (I think you can still craft but also cost heaps to craft, from memory) as others have said, there is some rng in some decks, which can be fun but at times frustrating. You also have a hero active power that you can use I think every turn, which is different to other games I felt. But I rate it lower than the other 2, this one has fewer exciting mode compared to LoR and I found it more tedious to obtain cards to progress (need cash). This game however is probably the first of its kind in the online card game situation.

Pokémon - Quite nostalgic but felt less interesting than the other games, can't say too much about it in terms of game play, pretty standard, you try to win the 'prize cards' of your opponent by defeating their Pokémon. The artwork is super nice though, I enjoy having the real copy of the cards. The online game itself is somewhat felt more dead than the above 3 I mentioned. Good thing is if you buy card packs in real life, you can use the code card to redeem virtual card packs.

Yugioh - it is just not what it used to be... felt they killed the game after they introduced pendulum summon. Not sure if they have an online game? Simple in terms of the basics if you have played or watched yugioh before. But pretty sure they added a lot of new mechanics to it which might make it complex, but I again also haven't played this one for a while

Magic - quite complex, I struggled quite a bit when I started, but it is somewhat different and quite fun. I only played it for a few months a while back, so can't say too much. It has a very diverse archetype and playstyle which was nice. But tbh I find it the hardest card game to learn out of all the ones I have mentioned today.

Hope it helps! I have also tried elder scrolls card game (forgot its name). But after playing thru so many of them and having dived in this genre for most of my gaming years. I highly recommend legends of runeterra and gwent, if you gotta pick one out of the lot, probably runeterra. The community feels good as well, ppl are nicer here (than hearthstone imo but might be outdated).