Realism in vocalsynth is down to multiple factors, but in terms of microphones, it's down to...
1. The quality of the mic itself and the interface behind it
2. The quality of the voice going into the mic and the output that produces
UTAU and the engines (resamplers) made for it are sensitive to the digital and analogue noise and sibilant signals that a mic/human naturally produces. As such, because it directly reads wav samples and interprets what went into them, it will feed back everything to you. Gahata Meiji, Namine Ritsu, etc. are recorded on professional studio mics in controlled environments with sophisticated interfaces that keeps the noise signal to an absolute minimum while only capturing their voices. The singers themselves approach their samples without any excess airiness that would cause the engine to produce what we'd interpret as "static" or white noise. The price for this quality is going to be significantly more than what your average user of this forum would pay/suggest. Many people here - if asked - would point to what are essentially low end (not even quantifiable as low end home studio) mics that all have their own flaws and have a significantly higher amount of noise (digital or analogue) due to the simplicity of their build. The least expensive low-end home studio mic option that I'd personally suggest (although
@bodytemp would be a better source of information on this topic than I) that would get you closer to the realism goal - as far as mics are concerned - would likely be the sE Magneto by sE Electronics. It's a good home studio mic for those on a small budget ($112 retail) and XLR interfaces to power it can come as simple as the Icicle by Blue ($49 retail - it's low end and really basic - seriously, there's nothing more to it than the cable and a knob that controls the volume, so analogue noise will still be present, but the results will generally be clear without the noise making a high impact).
As Cdra said, it's really all down to the budget you can afford and are
willing to spend for the quality you want. If you're not willing to spend more than, say... $100 from a low end brand and the interface is little more than the mic jack or direct USB, then you're going to get the quality that that entails (higher than the mic on the average laptop, but much less than Namine Ritsu or Gahata Meiji, and also less than a mic powered by XLR).