This is going to be HUGELY subjective based on what your needs are.
Do you need to record a full band, or just yourself? You'll want to consider how the acoustics of the room affect what you're doing and whether you have enough mics, preams, and channels on your recording device. Do you have some particular instrument in mind? You'll want equipment that relates well to that instrument. Will you need an isolation booth? Will you be recording direct? Will your neighbors throw a fit? All of these things matter.
You can build many fantastic setups for less than $10K and the "best" one will be the one that serves your needs the best.
The things I'd consider useful for any project studio would be:
- At least one really nice sounding preamp or interface. I'm partial to tube preamps.
- One dynamic and one large-diaphragm condenser mic. Between these two you can make a good attempt at most simple/straightforward recording situations. If you're going to be doing most of your recording through these mics (as opposed to direct), don't skimp on these. Choose mics suited to things you want to record. Make sure your preamp has phantom power if you have a condenser.
- One nice pair of good nearfield monitors with a reasonably flat response. You want to mix on these, not host a party with them (that's what home stereos are for!)
- A great DAW. Sounds like you want Pro Tools, which is an excellent computer-based DAW.
- Micstands, XLR cables, maybe 1/4" instrument cables, and other such accessories.
- Great friends to work with.