For music to be "timeless", I think it has to fulfil one or both of two categories: -
1. The artist has to be at, or near, the top of their game in terms of real talent. Whether they get mainstream recognition or not, game recognises game and names live on through that.
2. The song/album has to make the listener feel real emotion. Not just temporary "right time, right place" emotion, because that only makes for a throwaway pop song that's forgotten inside a year.
Well, if you're talking non DJ music, then yeah Beethoven, Mozart, Bach, Chopin, Rachmaninov, etc. I love Mils Davis on the jazz side. Blues is just too tough for me personally to really get into without seeing it live.
From a rock tip, Zep, the Doors, the Eagles and dare I say Nirvana?
Public Enemy from a hip hop point of view.
I think all these artists not only were freakishly talentd, but their music in many scenarios was so far ahead of its time that that's what makes it be able to stand the test of time.