I always thought the best album of the 90s (which I love and both of my parents love) is Oasis' (What's The Story) Morning Glory. I mean it sold 5.1 million copies for a reason.
Blur was always better... it just took history to prove it (each successive album was better than its predecessor, in my opinion). And while Definitely Maybe was a better album than Morning Glory. The latter was a definitely a worthy follow-up.
Oasis, unfortunately, were too closely linked with the Beatles. They finally freshened their sound up when it was already too late... Don't Believe The Truth was where they finally got it right since Morning Glory.
However, Oasis vs. album sales... look at the sales figures on Be Here Now.... and then listen to Be Here Now... and listen to how badly Noel hated it. That album helps to prove that sales figures are not representative of how good an album really is.
Finally, Oasis target audience are the masses whereas Radiohead never intended to please their fanbase or widen it for that matter. They deliberately tried to sound less commercial as time progressed. Instead, they chose to organically progress their sound and compositions. The reward comes upon repeat listens. Kid A and In Rainbows are shining examples. OK Computer was their Revolver (in my opinion, again) as it appealed to the masses with their hits (Karma Police, Paranoid Android, No Surprises, Let Down) while progressively moving forward (Climbing Up The Walls, Exit Music, The Tourist).
If you had to pick 5 albums that defined the 90s - this is one of them. (Nevermind, Achtung Baby are two other obvious ones)... arguably Morning Glory could make that list (even though Definitely Maybe is the better album) along with Automatic for the People.