there are thousands to choose from...literally. over the past year, my favorite all-time album has become, due to it's greatness from beginning to end, Raphael Saadiq's "The Way I See It". Awesome album.
I think it's hard to label what the best album ever is, considering the different genres and what not. For rap, I gotta say 7 day Theory, and for rock I'll say Nevermind by Nirvana.
Other better debut album contenders - "Ten" by Pearl Jam, "Hybrid Theory" by Linkin Park, "Bleach" by Nirvana, "Never Mind The Bollocks" by The Sex Pistols, and "Led Zeppelin" by Led Zeppelin.
I'd take any of those over "Definitely Maybe". It's not even the best Oasis album.....that would be "(What's the Story) Morning Glory" IMO