I agree with the off-putting Will Smith factor - don't rate any of his films although I Robot is just about watchable.
2001 is a bit of a Marmite movie - you either love it or loathe it. Many people find it overlong and tedious and both are fair comments in the light of subsequent sci-fi movies, which are often very fast-moving (although Alien is quite slow to really get going.)
The thing with 2001 is that it's quite a pioneering film and neeRAB to be viewed from the perspective of 1968 when sci-fi movies were mostly small budget affairs and no-one really took the genre too seriously. Sci-fi was the preserve of drive-in movies with eye-catching exclamation-marked titles like "Mars Attacks!" or "Invasion of the Giant Spiders!"
2001 changed everything: an ambitious film with a large budget, elevating the genre to an art form - especially given the complex narrative, the groundbreaking use of special effects etc. and the use of a classical music score (many films before then featured original scores and didn't make extensive use of existing classical music.)
It is, however, very slow and there is very little dialogue; it is essentially a visual experience more than anything else and these days the visuals, good as they were back then, are dated to some extent.
However, like Citizen Kane, it is the use of the tools of cinema which elevates this film above many others, another example of the cinema as an art form and this is why I feel it deserves its high stature and esteem.