This thread is interesting, and again probably shows my age (22, for the record--I think it might make this list more interesting if I include that) a bit. But I'd love to pitch in, with some additional "notes" thrown in:

The first TAPE I ever bought was the Monkees' Greatest Hits. I wouldn't place them on my list of top 5, but, sheesh, considering that was the start, is it any wonder that I love this type of music? And keep in mind, I was buying this tape in, like, 1986.

1) Billy Joel/Elton John. Billy Joel is the first artist that I ever owned ALL the albums from. Like many, I saw he and EJ as two artists that were of a piece (with each taking a decidedly different tack, of course, but they still had similarities) so I was into both at the same time... this was like elementary/middle school.
2) The Cars. This really defined my move from what were my two separate interests--alternative rock and classic rock--into a more poppy mixture of the two. The second band I bothered to buy all the albums from, and I still haven't tired from them to this day. For the record, around the time I was getting into them (1993-1994) they were about the LEAST cool band to like. I find it funny now that I know hardly anyone in my age group who doesn't own some Cars.
3) Blur. The mid and late 90s (late HS/college) were defined by Britpop, and no band did it for me like Blur. Anyone on this list who raves about the Kinks deserves to buy themselves a Blur album--I'd recommend "Parklife" as a starter, though any of their mod trio discs--"Modern Life is Rubbish", "Parklife", and "The Great Escape"--would do the trick. The three work extremely well together, too, in that each is a bit of a class study of modern life in the UK (in order, they examine the poor/working class, the middle class, and the elite). Realistically Blur's influence on me was augmented by the many, many great groups to be around at the same time. I really needn't tell you about them; I'm sure most of you know.
4) Fountains of Wayne - While I loved their debut, it was "Utopia Parkway" in 1999 that tied it all together--Britpop, my new wave fascination, and good old American power-pop and made me seek out places like NotLame and the audities community.

Okay, that's only four. Maybe I should've included the Monkees?

--Jason