Best 4X Space Sim bar none! Excellent in almost every way. You can really lose quite a lot of hours playing X3 and the Albion Prelude is a great continuation of both the plot as well as what I would consider the main open-ended game. Albion Prelude features just about every fix or tweak you could ask for and as a result is extremely polished.
Do keep in mind how this game rewards spending time. It's not grindy in an MMO style, but one can easily imagine how 'trading from bottom to top, earning millions' will simply take time. People who want an easier start should definitely start with the Humble Merchant setting and perhaps try their hand as the stock exchange market to get their first millions. Then after that you can essentially do whatever you like. The game allows for multiple save games, so there's definitely room for experimentation. Some things, like setting up an automatic trade route do often require specific space ship software systems, same goes for managing space stations you can buy and run. There's a slight learning curve to expect there, certain sectors in the game are easier / more difficult to get profitable stations going.
Exploring the X universe in X3: Albion Prelude is awesome. If I actually had more time to play, I'd probably easily spend hundreds of hours in this game. The open-ended nature and the freedom of choice is great.
(Also, do not buy X: Rebirth. So far X3: Albion Prelude is superior in every single way imaginable, except for perhaps the graphical fidelty. X3: Albion Prelude still is a great looking game though.)