The reason, I think, that The Beatles are so good is down to the vast variety of styles in their songs. Perhaps you want a sad kind of song, then you listen to While My Guitar Gently Weeps, or a happy one, then you listen to Here Comes the Sun, or perhaps you’re a crazy person who likes weird avant-garde stuff, then you can listen to Revolution 9.
Plus, aside from some of their earlier music, The Beatles do have a very particular style that encompasses all of their songs; the 'sound' of their music is very unique. Partly and this even includes the earliest things, I think this is down to the Liverpudlian accents, something not very often heard in music.
But, other than just mindlessly praising The Beatles, I'll use this entry so that I can tell you about some of their really good but lesser known songs. I won't give you links, as I don't think I legally can, but search for What's the New Mary Jane, Not Guilty or Free as a Bird, I'm sure you'll like at least one!