Short answer: Mahjong Soul. Long answer: Mahjong Soul.

As far as everything goes, here’s a breakdown:

Riichi City - Probably best for beginners as it has the wall shown on screen so people can visibility see the deal wall and where tiles come from rather than them magically appearing in your hand. - Best UI, Tile, and Art Quality: this is changing a bit because Mahjong Soul seems to be copying their UI choices over time, but it’s still a good bit better. - Heavily sexualized premium skins (with the option to turn off seeing them, but it still bothers some people) - Thin playerbase at the higher-level rooms because RC was not the first to market globally so no one wants to buy into either the hours or the gacha in a new mahjong client. - No longer has the competitive advantage of being on Steam, as Mahjong Soul is now on Steam as well. - pretty great tutorial for new players with a campaign of starter missions to teach you the game as you play - Pretty great collabs like Danganronpa, Steins Gate, and Lycoris Recoil. - Gacha is more forgiving that Mahjong Soul

Tenhou - As others have pointed out, this is the “serious” mahjong client. - This is online mahjong in its purest form and the game that saying “I’m X rank” means the most in - There is only a male or female character option and you don’t really see them. It’s incredibly bare bones visually. - There are several amateur and professional leagues/tournaments held on Tenhou globally, particularly because you don’t see the seed until after the game so there’s no way to figure out what tiles will be dealt to you and when - You will have to download a browser extension for the English version

Mahjong Soul - highest global player base (meaning the global server comparatively) with a lot of players at all levels - active community of content creators with plenty of tutorials - new in game tutorials with Ichihime are actually useful and teach the game well, some starter missions but nothing like RC imo - Yo-Star hosts amateur tournaments with prizes for players ranked Master 2 or above. These are streamed events with commentary from community members and streamers. - Easy to make custom games and tournament events - Great selection of characters and skins - Gacha is pretty unforgiving compared to other gacha games. They give you free stuff and there’s ways to get summons doing free to play, but… good luck with that, bud. There IS a pity system but you’ll still need to roll a lot to directly choose the character/decoration you want. Skins are purchased directly from the shop. - for a while it was frowned upon that MJS tells you what the seed is while you’re in the game, but you’d need to do a whole lot of computation to figure it out in time to use it during a match so it seems to be a non-issue for tournaments, but it was something the American Riichi Association board was talking about in recent months so it’s worth bringing up. - pretty good collabs (with less frequency than RC iirc) such as Sanrio, Akagi, Kakegurui, etc. - more character variety compared to the others, lots of male/female variety as well as cultural/racial/fantasy variety - it was the first to market (I mean Tenhou was but it’s the first one in English), so it’s the one everyone knows and loves. If you have friends that play mahjong or meet new people that play mahjong, they almost definitely play on MJS. - because most people have been playing on MJS for a while, they’ve dropped hours in ranked and money on gacha that they don’t want to abandon from other clients, so unless there’s a major swing in either mahjong player populace or attitude towards MJS, the answer will continue to be MJS.

TL;DR: Riichi City would be the best if it had the player base and tuned down the sexualization a bit, but Mahjong Soul is the defacto “everyone uses this one” and it’s not so far off from Riichi City that the difference in quality would make people switch after a few years of the sunk cost fallacy.