These are my thoughts on choosing the right microphones for a variety of scenarios, based on my way of approaching recording projects.

First of all, here are a few of the fundamental observations I have found helpful:
* The more live a recording situation is, the easier it will be to make it sound live. Simple ;-) * The fewer microphones are used in the mix, the clearer the result will be. This is due to...

+ Phase issues: the fact that all sounds occurring will to some degree be present in all microphones, but at slightly differing time; they might also be colored due to non-linearity in off-axis pickup.
+ Additive background noise, which can also cause problems when spots are panned to a specific location.

• Music is a temporal and spatial event. Musicians generally move (their instruments) and this causes a movement of the sound source which dynamically interacts with the recording space.
• Never forget the importance of the microphone preamplifier & the ADC!
• There is a distinct difference between far and close-up use. The character of a microphone's sound is much more significant in the latter case.
• A microphone does not know what it is recording. Be suspicious about type-casting (guitar-mic, snare-mic etc).

Based on the above assumptions there are a few questions to be asked about what you wish to record.

Hauptmikrofon / main mike setup

You need a set of closely matched high quality microphones for this purpose. The key features to look for are neutrality, resolution and quietness. You don't want off-axis sound to be coloured, especially when using omnidirectional microphones in an AB-setup.

Blumlein-setup

The Blumlein pair requires two fig-8 microphones. Keep in mind that all bidirectional condensers exhibit (a pronounced proximity effect when used close up and) a significant low-frequency-rolloff in the far field. Interestingly ribbon microphones don't necessarily share that characteristic.

>= 88.2/96kHz-recordings

If you plan on recording high resolution audio it is advisable that the microphones (capsules and capsule-preamplifier) reach up to half of the sample rate, preferably in a linear fashion. Check out the Nyquist frequency for details.

Ambient microphones

Since these microphones will be way out in the reverberant field where the sound pressure level is relatively low, low self-noise is desirable. It is when you want to make the most out of directly approaching sound that the use of boundary layer microphones is indicated.

Spot microphones

Spot microphones is the area where recordists can get most creative. Any microphones' sonic character becomes more pronounced when used close to the sound source, especially with directional characteristics.

The blumlein records microphone closet
* 1 * Earthworks QTC-1 (QTC40)
This used to be one of a set, but a musician literally killed one in a session --tripping over a cable and making the mic crash tip-down. I often use it as the mid-channel for an M/S-setup. * 2 * Earthworks QTC50 (matched pair)
Optimal for high-resolution AB-recordings. Their miniature capsule is extremely linear and amazingly fast. * 2 * Elation KM201 MSP4 (matched pair) - 2 Capsules: cardioid & omnidirectional
A good general purpose spot microphone. I have also frequently used it as an AB-spot within choirs when recording oratorios. * 2 * DPA 4060 (matched pair)
A beautifully unobtrusive microphone. It does have a self-noise of 23 dBA but works great as a close spot inside a bass, auto harp, udu etc. I also frequently used it as a boundary layer mic taped under a grand piano. * 2 * Oktava mk102 UM-mod (matched pair)
The United Minorities tuned Oktava cardioid is fabulous for ORTF-recordings, as a mid-channel for M/S, and as a close mike for extremely quiet sources. * 6 * Rode NT-5 (3 matched pairs)
This inexpensive microphone is a good spot for many applications.
Currently all six of these microphones are the basis of my XYtri-setup. * 1 * Rode NT-4 (matched pair)
Using the same capsules as the NT-5 this microphone is quick to setup and does a good job. I have used the battery-powerable mic as an external microphone feeding my Mini-DV-camera for many interviews. * 2 * RoyerLabs SF-1 (matched pair)
A beautiful ribbon microphone. Great for sonophile Blumlein recordings. Also a very nice spot for many overtone-rich instruments: gaita, trumpet, violin-overhead, ... due to it's gentle high-frequency roll off. * 2 * UM-1 (matched pair) - Switchable: cardioid, fig-8, omnidirectional
Since I have received this United Minorities microphone it has been the staple for many of my recordings. It is very linear, extremely accurate in the time domain and eminently usable in a multitude of recording situations. I love it!

Disclaimer: I have occasionally used the famed microphones by Neumann and Schoeps, but I never felt the need to buy one of them. Equally good microphones are available --at a more reasonable price range. Let your ears be the judge :-)