The Beatles are great for many reasons. They really did influence music. They're probably the most well-known band of all-time. I think "Yesterday" is the most played song ever.
They've got variety. Lots of good pop songs, some good rock songs. They appeal to many different audiences.
Look at the lyrics. The lyrics are fucking strong. I have yet to see somebody write lyrics as well as Lennon and McCartney could. Hell, even Harrison. And it isn't just a couple of their songs, they were puttin' out great stuff all the time.
What I consider to probably be one of the most important reasons they're great is their voices. I listen to many of the bands today, and sometimes I can't understand what the hell they're saying, or I think "What the fuck...anybody can sing like that." Sometimes it's just untolerable. Harrison, Lennon, and McCartney have amazing voices, and Ringo wasn't that bad.
I'm done, I don't have time to go into detail about reason why I like the Beatles and think they're the greatest band of all-time. All I know is that they're different than most bands today, and I think that's great (I don't really like the way rock went after the 80's).
And Oasis....damn, I like Oasis, but I don't think they're deserving of a spot that high. They haven't really had that much of an impact, and their music isn't really that outstanding. Great (I'd rank them in the top 25) but not 11th in my eyes.