In your opinion, what Prince album sounds 'dated'? I mean: very much reflecting the era in which it was recorded, sounding a lot like other music around that time, even soundig a bit 'too' oldfashioned today?

My vote would go to For You. IMO its very much a 'seventies' record. The sound of the faster tracks often reflects the discomaterial that was released by other artists around that time (Earth wind and fire, Michael Jackson), while the ballads echo jazzy singer-songwriters like Michael Franks (So Blue, Crazy) or polished late seventies soul (Baby).
Instrumentaly this record sounds quite date becaused because the rhythem section doesnt sound really distinctive. It sounds almost exactly like the the blend of bass and drums used by so many popular soul / disco-artists around that time.
Its true, Prince tries to do something new by replacing the parts usually played by horns in this kind of music, with synths, but those synths often sound very dated when you listen to them nowadays.

The opposite of a dated sounding album is a timeless one. For me this means an album that still sounds 'fresh' 20 or 30 years afters its orginal release, an album which has a timeless quality, which doesn't show its age, an album that isn't really copying the sound of its era, but one that transcends time.

For me the choice would definitely be Parade. Apart from Anotherloverholenyohead, which uses a bit of 'eighties' sounding synths, its an album that doesnt sound like typically 'eighties pop' at all.
I think a lot of this has to do with the fact that Prince is 'using' music from so many different eras as an influence: romantic classical music from the beginning of the 20th century (Venus de Milo), French chansons from the twenties (Do U lie) cabaret music of the 'interbellum' (Under the cherry moon), jazz from the fifties (Girls and Boys), Psychedelic pop from the sixties (Christopher Traceys Parade, I wonder U), Sly Stone-like soul-rock from the sixties / early seventies (Mountains), Singer-songwriter folk from the seventies (Sometimes it snows april), and more avantgardistic sound experiments from the eighties (Life can be so nice, Kiss).
Maybe even more crucial for the timeless sound of Parade is IMO that he does't deliver strict 'pastiches' of the forementioned 'styles'. The way he arranged these songs still sounds fresh because the way he mixes those influences in his music in an original way. A lot of that has to do with the emphasis he has put on the 'beats' of those songs, not copying the bombastic drumsound that is so common for many songs from the eighties, but going for a distinctive sound, that is more 'dry', subdued and IMO very timeless. He was an originator in that time when it comes to arranging and producing 'beats', not a follower (like on many of his songs from the nineties).
But its not only the sound of the beats, its also about the arrangements he creates on this album: Christopher Tracy's Parade could have been a Beatles song from the sixties, but the emphasis on the beat gives its a 'funkieness' the Beatles seldom achieved, The horns on Girld and Boys reflect fifties jazz, but the way he combines them with eerie synths in the background give the song an avantgardistic touch, Kiss is in essence classic James Brown-funk, but he strips it down in a way that it becomes almost 'abstract music', even the pure, folky sounding Sometimes it snows in april has an 'arty' edge: he keeps the little 'accidents' created by acoustic instruments (cracking of guitar-snares, too much reverb on the piano-strings) in the mix, while its usually common to mix those things out of the music.
The elements i mentioned here make Parade a record that still sounds fresh today (21 years after its original release, geeze, is it really that long ago?....) and i think it still will do 20, 50 or maybe even 100 years from now.

So, what do you think? what is Princes most dated sounding album? and what album is his most timeless?

I would agree with'For You' 100%, timeless sounding could be a few, I think, but off the top of my head I will go for 'Lovesexy' as a timeless album. Don't think it sounds like anything else, certainly not in '88.