I don't think it's obvious. In terms of player count (which is how I would personally rank success), it is not the most successful by a long way: https://en.wikipedia.org/wiki/List_of_most-played_video_game...
However, games like PUBG and Fortnite are free-to-play (PUBG is only free on mobile?) so in terms of actual sales, you could say GTA is more successful. Still not sure I'd class it as an "online shooter", though.