The Beatles, which, through The Beatles: Rock Band got me truley into music [and playing drums], though I had seen Springsteen 4 times previous to this...

However, this eventually [and I mean eventually] gave way to a greater musical spectrum, taking me eventually into Pet Sounds, which then months later took me to the original UK Best Of LP [I didn't know the hits!] and then on to the overall picture... So yeah,
The Beach Boys.
You could also throw in
The Kinks, but obsession lies within the 'Kontroversy' to 'Muswell Hillbillies' era. After that I do still get the LPs, but I wouldn't call it obsession...
Definitely [no maybe]
Oasis and the related solo acts. Got into these just as Noel's solo effort came out and absolutely dig their work. Probably know 80% of the lyrics too

My most recent one would have to be
Weezer, probably because you can attach Rivers and Brian relatively closely in a sense. Love all of their sometimes questionable output - Weezer Cruise 2014 here I come!
I suppose on a less obvious note you could throw in
Michael Dermot and
Wyatt Funderburk. Internet based artists with only 2 and 1 albums respectively, but everything both have released upon this world is absolutely perfect. And I have played their music A LOT. Definitely seek them out!
I 'spose that's it...