One could look at this from the perspective of: What will I use the fonts for?
The two sources given by Tomence are excellent ... but in all likelihood you will still need a good sized library of "classic" fonts for what will be one type of work you will create. That being designs where you are matching preexisting designs and font selections but are not being supplied with a vector file by the client. All those boring fonts like Caslon, Baskerville, Garamond, Helvetica, Copperplate ... and yes, even Brush Script.
When you are being asked to create a design with no up front conditions, good quality but less widely used fonts will give your work a style that will please many clients. The
SignFonts offerings are probably the best bang for the buck you will find. But when you are asked to match up to something designed beforehand, you will need the more commonly used fonts in reserve.
These days, there really aren't all that many collections around. Most manufacturers have taken to selling individual downloads due to widespread illegal duplication and piracy. My usual recommendation is to purchase a license for CorelDRAW ... even a used version that can be several versions out of date. This is because it includes the entire Bitstream library from around 1994 when Bitstream sold a license to Corel in order to avoid their own bankruptcy. Corel has been giving it away for free with the purchase of CorelDRAW ever since.
What that doesn't cover you will be left to acquire from the internet sites that sell them individually, but you will be covered on the majority of jobs which involve duplication existing font selections.