My answer would be Fallout 76 if I hadn't come around to considering it a better game than Fallout 4, another game I purchased day-1.
I guess it's been a long time since I bought a game that I consider to be genuinely, like, SHIT though.
I mean, the games I've bought in recent years that I don't like, or dropped because I didn't enjoy them, are:
Valkyria Chronicles 4 - ranking system prioritizing turn count over everything else, dictating that I play not like a strategist but like a fucking madman...
Pokemon Sw/Sh - makes games that are too easy look too hard in comparison
Fallout 76 - broken as fuck at launch, have had more frustrating experiences with the game than I can count
Fallout 4 - lame, all the time, every time, not even mods can fix this
Battlefield 5 - sublime gunplay and game feel but the patches over time were Uncharted 2 MP levels of "damn why'd y'all have to go and do that" and Firestorm was a misstep
Looking back... I guess I just know how to pick 'em. The worst game I've ever bought was... Project X-Zone. Yeah it's a fanservice game first and foremost but you don't even gotta use your brain and for a tactics game that's regrettable. There are so many worse games that I guess even X-Zone hardly ranks against some of the shit we're bound to see up in here.