An SSL console. I was working on an SSL 4000G today (along with a Teletronix LA2A, also seen in the background which is not turned on - because you want to save the tube for when you actually use it).
The SSL console is not cheap.
Fortunately you can buy 500-series preamps and compressors (clones) that will yield great results, if you just want to use one vocal mic.
Edit: Oh, I haven't posted here in a while and I just noticed I still have the SSL bus-compressor as my avatar. Nice!