Well, you're right about the elements that go into the mix bar one - cost. So here's some discs I love, with cheap alternatives in some cases!
I assume you mean classical in the widest sense. Also, I'll try and keep it small and relatively accessible. These are some of the "warhorses", with a few others.
Bach - Brandenburg Concertos
Concerto italiano on the Naive label - fresh, lively performances in marvellous, clear sound. Not cheap though.
Bach - St Matthew Passion
Phillipe Herreweghe on Harmonia Mundi. This is, IMO, the best piece of Western music ever written, and the best "big band" recording. Great sound. There is a very good version on Naxos also that goes for a lot less, if you are on a budget. This is *not* an accessible piece of music, but it is one that will reward you for the rest of your life if you make an effort.
Beethoven - Symphony no. 9
David Zinman on Arte Nova Classics is good and very cheap. Roger Norrington for period instruments. I don't own a definitive recording of this one, so I'll let others chip in. The music, though, is among the first rank of the classical repertoire, whatever your preferences.
Mozart - Requiem
Phillipe Herreweghe on Harmonia Mundi again. Currently available cheap as part of their 2006 Mozart Edition. An amazing work: all Mozart's rich, tuneful genius properly channeled through study of the old master, Bach. This recording on period instruments does not hold back on chunky, throbbing power and the choir is, as always with Herreweghe, superb. The alto soloist is a bit crummy, but her solo parts are quite minor in this anyway.
Monteverdi - Vespro della beata Vergine
Rene Jacobs on Harmonia Mundi (Robert King on Hyperion is also supposed to be amazing, though I don't have it yet).
Another choral work, one that seems to look back from the end of the renaissance to every strand of Western music before it, and wind them all together into a beautiful tapestry that also innovates and looks to the future, with an emphasis on the simplicity and power of singing.