How many songs does it take? All of them. The discography. If you've written an album or even a song that loses me, then you're not a band that I invest in.
I have no need to play "my band can beat up your band", so this isn't the place to name my favorites specifically. I certainly appreciate how some fans are alienated by my favorite bands' stylistic or personnel changes (just as I am by many bands as well). Any band who survived to write a discography worth investigating didn't escape being forced to make artistic choices against ugly, industry trend-implosions or internal ego clashes.
So my favorites become the ones whose responses on that mark certainly challenge me, but also thrill me and change me, and ultimately fill me in some way.
By their growth as lyricists, composers and arrangers, I find camaraderie digging back into their discographies, reflecting on how I grew as well, aging right along with them. So their musical evolution becomes the joy of the whole thing, rather than needing to make apologies for an embarrassing album here or there.
By definition then, my favorites are those wherein such band evolutions only reinforce my studied interest and investment in the entirety of their histories, such that I became a kind of student of the band. It's extremely personal, but therefore quite indefensible, obviously. I'm simply fulfilled to trace the development of their entire histories -- from rediscovered, scratchy, garage demo cassettes; to swansong; and finally to memorial tribute boxsets.
Such artists' life catalogues speak to me personally -- musically, lyrically, even philosphically. Not many bands do that for me. A handful do.
That's certainly not to say I don't enjoy other band's music, classic pieces, important songs. But all of that's just jukebox at best, or wallpaper at worst.