Yep. I can generalize about a hypothetical group of games that might be perfect for me, but some people would hate them or find them mediocre. But, because it's fun, I'll take a stab at describing a game that I as a player would find perfect.
My perfect will be different from your perfect.
First of all, I like linear games with a lot of story and a distinct ending, though a "new game +" is a nice way of combining replayability with an ending. Some sandboxy elements, like house building and character appearance customization can be a lot of fun, but only games that lead the player through by regularly providing goals and story context to make those goals meaningful really work for me. So if there's a story, the question then becomes what should that story (and related gameplay) be about. Well, I like stories about personal evolution and technological/cultural evolution, stories about worldbuilding mysteries, stories with magic, stories where the main character solves other people's problems, stories where the main character breeds monsters or plants to discover new hybrids (yet more evolution), and stories with romance. This suggests RPG and sim play, possibly also tactical play, and I like arcade and speedpuzzle play too, as well as collecting sets and soloing dungeons. I do not like turn-based non-tactical combat so I would not include that under RPG elements.
To provide the most scope for evolution to happen within the game, I think it would be good to start with the playable character being quite blank/generic. Their clothing would be limited to gray or brown rags or a very boring gray or brown uniform. They should also have boring hair and faces, no tattoos or jewelry, no race or class chosen before the game starts; since those kind of choices are the fun part they should all be in the game itself, as rewards. The game could start either with the main character being marooned on an alien planet with no nearby signs of other sentient life, or as a student enrolling in some kind of mysterious fantasy or sci-fi academy that serves as a gateway to a fantasy or sci-final world once the student demonstrates enough basic skills to be allowed to explore on their own.
So, if I'm marooned on this alien island, the first thing I would have fun doing is walking around to reveal all the immediately-accessible parts of the island on the map. At the same time I'd be looking for gatherables with which to start crafting my way up the tech tree of civilization. So that would be about exploration with the world itself serving as a test of the player's ingenuity; the distant dangled reward would be the hint that the player will find some other people after they master their island and become able to travel. The school version on the other hand would have more of a feel of socially imposed challenge and testing, with the player striving to be good enough to gain rewards that would mark them as competent and adult, such as cooler clothes and weapons, possibly a cool mount, and the ability to explore outside the school. In both cases the player might also gain some initial magic or combat abilities. Shapeshifting is possibility my favorite kind of magical ability, but elemental magic is also pretty cool, as are fancy weapon techniques, or captured and bred pet monsters could have various animalistic abilities. Real-time combat and tactical combat are both fun; for a realistic game development project I'd pick on or the other, but since this is a hypothetically perfect game I'll be greedy and say it should have both.
After the player has been able to learn the rudiments of combat and crafting and customize their appearance a little, then the game world can open up for them. Some mysterious ruins to explore which hold hints about the worldbuilding as well as two or more rival factions that want the player to build reputation with them would be a good start. Also some romanceable NPCs, who give out quests by which the player can earn their good opinion. If it's an MMO, they can now try PvP, minigames, and then begin a journey from the local capital (where they have arrived after completing the beginner area) to the world capital. There's a whole list of common MMO features that I would or wouldn't want in a perfect game, but it would be boring to list them all; the most important one is NO forced group play. The other people are there to have someone to show off your cool loot to and to make the world feel alive, but it's not all that fun to actually have to work with them. Overall, the player would continue to customize their appearance, climb their tech tree, develop their property/house, breed pets/plants, earn more magical techniques, collect collectibles, earn achievements, play minigames, climb the PvP rankings, and explore the game world to find more clues about it's mystery.
Edit: Oh yeah, and for bonus points let's sprinkle the world liberally with adventure-game style puzzles. And give the player a gazette and level/treasure completion map ala Vagrant Story to keep track of their progress at finishing off all the content (achieving all the achievements and being rewarded for them) in the world.