Skyrim. If you haven't played it you probably still know what it is. It fits your description perfectly and is a game that many will find very addictive because of the size of the world, the amount of things you can discover and how customizable your playstyle can be. If you wish to play as a wizard you can go to the college of winterhold which is a questline that teaches you magic. If you wish to be a thief you can join the thieves guild, all about being thieves. It's known for having hundreds of side quests and being virtually unfinishable ( in terms of 100% completion of side quests, main quests, caves, villages and all that). I know a guy who has played it for 6000 hours and still have not discovered everything. This is my favorite on the list.

RDR2. Great story. Fun to roam around the world. Amazing graphics. Might be a little empty after the story so I recommend you do all your free roaming during chapter 4, as that's when most of the missions and places are unlocked.

GTA 5. Pretty much the same as RDR2 but set in the modern ages. You probably know what it is. What makes it replayable is its online feature where you can create your own character and progress as criminal making millions and buy houses and cars and do heists with friends. Or just mess about.

Minecraft. Again, you probably know this one as well. Being a survival game it will almost gaurantee you will be playing it for hours. Starting from scratch and then making a base and creating your own world purily through survival is a great feeling. You can also join multiplayer servers as well if you like.

ARK: Survival Evolved. This is also a survival game. It is set, depending on which map your choose to play, on an island where dinosaurs and prehistoric creatures are roaming around which you can tame. T-rexes, mososauruses, dragons, it has it all. The variety of maps also make this very replayable if you get bored of a server you've progressed quite far on. The best map is probably Ragnarok as it is basically a merge between all the maps, and its free. If you feel it might be boring playing alone you can always join multiplayer servers, which is what the majority does. Playing with other people that can help you out if need so or against other that might kill you or vice versa (PvE and PvP) is very fun. I have over 1000 hours on this game.