Best PC game ever ? I guess it all depends what games you like, right ?
Best 4X game ever ? Now, that's a better question.
IMO, the best 4X game is MoO2. Why ? I prefer space strategy games than tactical games like Civ. You fight for control of planet, not of cities. There is something so cool about controlling fleets of space ships and invading planets. The schema of thing is just so much bigger. As I said, this is mostly a matter of preference.
For those not familiar with the difference between tactical and strategic: In a tactical game, units position is key on the map. Choose where you will stand your ground for defense will make you win a lot of battle. Obvious example, fortify on a hill with infantry, cavalry on open terrain. Bad position and you're gone.
In a strategy game, the map is defined into area, like planet system or sector of space. Usually, there is no defensive/offensive difference between sector/system. The defender does have the ability to build defensive structure though. This is where a good strategy becomes vital. Control choke point in space and you'll win the game.
I still enjoy very much tactical 4x games like Civ, but I feel there is so much micro management of units. For the whole game, you have to micro-manage your worker because the automation routine is not adequate. As the game progresses, it become very tedious. You also have to micromanage cities to a certain extent. As your empire grow, you have so much cities to manage. The build queue is great, but happiness should also be automated. Why do you have to wait for a city to be in revolt to get the option to "give them some happiness". There should be an option that will garantee that your cities will never revolt due to population increase. This add micro-management and it doesn't add anything to the fun factor.
Also, units should have the ability to be grouped together. I think they will offer this in the extension: group a stack of units so they always move together. About time if you ask me.
Complexity is great, but it should add to the fun factor, not just make the game complicated and more time consumming.
My 2 cents