Hello everyone and Robbeasy in particular!
My favorite of the X-series is still "X2: The Threat", mostly for its unique atmosphere and ease of play.
I bought it multiple times (including the Super Box, and here on GOG.com).
As a disclaimer, I have not played neither "X: Rebirth" nor "X4: Foundations" due to their significantly higher system requirements.
Regarding your gameplay question, Robbeasy, "X2: The Threat" already features a very servicable flight control via mouse. Only in the menu systems, it is easier and much faster with the keyboard only (thanks to available shortcut-keys).
While the first X game lets you pilot only one relatively small ship, "X2: The Threat" started to allow either to buy or capture other ship models both small and large from almost any race in the game's universe! And the player can have a fleet of his own.
"X2" does still focus a bit on its story, but usually it is advised to do some side missions to afford some necessary upgrades (ship & equipment), because at regular intervals the main story missions peak in their difficulty!
Nonetheless, you are free, to do or proceed at your own pace and however you want. (I usually set myself goals to gain access to a certain ship class or respective ship model from a specific race, which the player has to befriend first. Or I go simply exploring all available sectors.)
Many do not like the cascading menu system in "X2: The Threat", but I prefer it due to how quick I can maneuver in it.
Despite lacking fidelity in its graphics and shaders (compared to the modern sequels), "X2" features small atmospheric details, that got lost in subsequent entries of the X-series!
I am in particular fond of the engine smoke trails and the more subtle lens flare effects, as well as of the ingame time references (naming scheme for years, months, weeks, days, hours, minutes and seconds). And both planet surface visits (similar to the ones in "Freelancer") and entering stations with minor ship classes for internal docking maneuvers were skipped in the direct sequels ("X3: Reunion", "X3: Terran Conflict" and "X3: Albion Prelude") and only returned in the two most recent games.
The newer games (starting with the X3 saga) provide an incredible sense of scale, though. Not only ship classes, stations, but also planets are really huge in their dimensions. But the user interface suffered a lot in the usability and ease of use department from the bigger focus on the sandbox elements.
The ships and stations do look more impressive and more shiny thanks to the advanced shader techniques. However, they do not show a damage model nor damage textures. The smoke trails are replaced by static cones of light. You can not look inside the cockpits anymore, neither your own (yes, no internal cockpit view camera for the player in any of the "X3" games!) nor from outside into the other ships! ("X: Rebirth" brought the cockpits back for the only playable ship, while "X4: Foundations" does feature multiple pilotable ships including cockpits again.)
And I prefer the space traffic system of "X2: The Threat". It is somehow different in the "X3" games, and seems to be less effective.
Otherwise, the majority of gameplay elements, such as manual trading, (automated) trade routes, piloting and commanding a fleet of ships and so on, are already implemented (albeit a bit more crude) in "X2: The Threat", thus a newcomer could get a relatively quick access to experience how the various gameplay types work in the X-series, deciding, if it is of interest or not, and eventually move on to newer games in the series.
Hence, I would recommend "X2: The Threat" for a first entry point into the X-universe before giving maybe "X3: Terran Conflict" or "X3: Albion Prelude" a go, and then jump on to "X4: Foundations" (if your computer can handle the latter)!
Kind regards,
foxgog
[Edit:]
Reading the other thread on "X2: The Threat"
crashing in mission six, reminds me to mention, that "X2: The Threat" has issues if you run it with higher or uncapped refresh rates! To avoid problems, it is advisable to limit the refresh rate (or fps) to at maximum 60 Hz--probably best by means of an additional program.
