The "ultimate" - as for "ultimate" - equipment would be something that kills the vocalist.
Besides that I would recommend clean drinking water and herb tea with honey in the vocalists wardrobe. Sometimes warm beer works wonders. Also coffee can help. Have all those available.
Next thing is I would make sure the vocalist knows how to work with a microphone in a live situation.
Then I would think about which mic to use based on what suits his voice and the music style.
Then I would consult about the quality of the PA system used, since there is no point if the "ultimate" signal leaves your FOH desk to be distributed over a shitty PA. (Unless you only care about the recording)
If I have time after making sure I have eaten well that day before the show, I would worry about the preamp, compressor, eq, mixing desk etc.