It's true that Oasis have never matched the heights of Def. Maybe, but then that's hardly a surprise. Noel had written all the songs for their first three albums before they even had a record deal. When they did get one, they chose what they saw as the best tracks and put them on Def. Maybe, the 'second best' tracks on (WTS?) Morning Glory and the rest on Be Here Now. They were both good records, but obviously not as good as their first effort.
Standing on the Shoulders of Giants wasn't great, but there was some decent stuff on there. Same can be said for Heathen Chemistry. Don't Believe The Truth was another great album imo. Some quality tracks on there, and the writing of the songs was shared around the whole band. So Noel, Liam, Andy and Gem all have songs on there. There's a new album out next year and I can't wait. No doubt some people will judge it as [Poor language removed] without even listening to it but there you go.
Green Day are just awful in my opinion. But then again, it's almost a whole different 'genre' of music to Oasis. So to compare them is pretty stupid. Music is all about opinion, and to some Oasis will do no right. But if you actually take the time to listen to their music, it's bloody good stuff.