For me it’s Overwatch. I really like the team based online multiplayer idea, but there are some fundamental flaws in Overwatch’s execution:

• First person and real time: this design decision means that players don’t know where or in what state their team mates are in, makes it harder to coordinate
• Rando matchmaking: putting strangers together and expecting them to cooperate while giving them no tools to do so (Overwatch 2 did eventually get the signal system, or whatever it’s called)
• Play of the game: celebrates individualism in a game about cooperating, wrong incentives
• Bad management: too many to list

So what’s a game you think has a great concept but poor execution?