Looking at my Steam, the game with the highest number of hours played, of which I would currently say unambiguously that you should avoid it, looks to be War Thunder. Among the reasons I'd tell you to stay away from it:

• It's a grindfest starting very early on.
• It's far too easy to lose as a result of what can reasonably be called bad luck.
• Unless you specialise, or throw real money at it, the fun, high-tech stuff is probably thousands of hours into the future.
• There's content gated behind "if you were not around when this was regular stuff, you will never get it"
• It calls itself an MMO, while there's nothing MMO about it. It's all instanced battles, with little to no world continuity as you progress.