All the WWI games I've played on this site (with a few that turned out to not actually be WWI games, but were on the WWI tag anyway). Occasionally good! Mostly... not.
To put it briefly: play Heartland, maybe play Cargo Breach, Major Doctor Bing Bong, and syuzhet (in that order of preference), and you really don't need to play anything else on this list. Two of the games on this list are broken, so it's possible they might be good, but I'll never know. If you can get them to work and recommend them, or have any other WWI games you recommend (on itch.io or otherwise), drop me an ask on Tumblr at aceredshirt13!
Not Yet Played/Completed
I mean, honestly this game looks awesome. Georges Guynemer finding himself in a strange fantasy world after crashing his plane? Incredible concept! However, I can't actually recommend it, because even though the GUI was translated into English... none of the dialogue was??? Not even English subtitles??? Obviously it's fine for a game to be in French - I'm not upset at this game simply for being in a language I can't speak. I'm upset because it claims to be available in English, and if I can't understand the storyline as an English speaker, it seems rather deceitful to say that.
This game is entirely in Chinese, so I didn't play it because I don't speak Chinese. Why am I mentioning it here, then? Because, despite the fact that the game is set in Russia, the creator openly said they machine-translated all the Russian in the game??? I wouldn't do that with the Chinese for a game set in China??? Bro??? It feels a bit disrespectful to the Russian language... Surely there was someone you could ask...