Impossible to pick just one. I can pick a few for totally contrasting reasons.
With a deal of thought, I've chosen four to narrow down from: By the Way, London Calling and, perhaps surprisingly, Weezer's Blue Album and Modest Mouse's We Were Dead Before The Ship Even Sank.
Starting off with By the Way, there's just so much to love about the album. John Frusciante is easily my all-time favourite guitarist. He's brilliant technically, obviously, but that's just scratching the surface of his genius. For example, 'Heartbreaker' by Led Zep has been played better, more technically proficiently, and more tidily by other guitarists, the original Jimmy Page solo has a feel to it that nobody else can match. In much the same way, Frusciante's relaxed, springy guitar style permeates the whole album, as does his underrated backing vocals. Add to that Chad Smith's machine-precision drumming, Anthony Keidis at his peak, and Flea combining with Frusciante's guitar to provide some of the best rhythms of all time, and the result is classics throughout the album: 'By the Way', 'Universally Speaking', 'Dosed', 'The Zephyr Song', 'Cabron', 'On Mercury', 'Minor Thing', 'Warm Tape', 'Venice Queen' and my personal favourite 'Can't Stop'. Just brilliant.
London Calling, on the other hand, doesn't have such an overarching style. Where By the Way's brilliance was in its simplicity and similarity, London Calling is a mish mash of loads of different styles. 'London Calling' is a brilliant piece of British punk, 'Wrong 'Em Boyo' is bouncy ska, and even disco gets a look in in 'Lost in the Supermarket'. What really shines through, though, is Joe Strummer binding all the elements together with excellent writing and strong, passionate vocals. Most of all, though, I think the album just invokes memories of sitting around on the roof of my friend's house, messing with guitars and eating cold pizza as 'The Guns of Brixton' drifts across the town after which it was named.
Onto my more leftfield choices. Whereas my first two would easily get onto 'Greatest Ever' lists, I'm not sure Weezer (The Blue Album) would, necessarily. It came out a year after my birth, and in my opinion totally changed the face of music in the nineties. Before Weezer, you had the likes of the - undoubtedly brilliant - Kurt Cobain singing about pain and suffering. The music was dark, meaningful, filled with sad emotions and hurt. Weezer came onto the scene and turned that on its head; their music was about just getting together with your friends and making cool noises on your instruments. It was the antithesis to Cobain. Where Cobain's Nirvana were brooding and troubled, Weezer's songs were about how someone they knew looked a bit like Buddy Holly. Even a song with the title 'The World Has Turned and Left Me Here' managed to come out joyful and melodious, in Weezer's trademark power pop style. As a standalone album, The Blue Album is excellent. When considered in context, it is truly wonderful.
And finally, the most bizarre - and most recent - of all my choices, We Were Dead Before The Ship Even Sank. Modest Mouse are one of my favourite bands, thanks mostly to the inventiveness of Isaac Brock's lyrics and the range of emotion and meaning they can invoke within the same style of indie, occasionally folky rock. Much like RHCP in By the Way, the style and mood of WWDBTSES can range from the whimsical ('Dashboard'), to the sarcastic ('We've got Everything) to angry ('Spitting Venom'). Most of all, though, it just has some immensely catchy tunes that make you want to dance around like an idiot, and if that isn't the sign of a good album, I don't know what is.
Out of those four? By the Way wins, I think. Can't Stop is a masterpiece, and possibly my favourite song ever, bar none.
That's enough for now. I'm tired as ****. My brain is nagging away at me, since I've likely missed out some absolute crackers of albums. The Beatles, for example, with Revolver, Sgt Pepper's, and the White Album, or perhaps Bowie, The Jam, Dire Straits, Pet Sounds... eh, I'll think of some more in the morning.