I'm allergic to silver...So I prefer Gold over anything else, really.
to me, nickel sounds like crap (no offense to anyone who uses it or likes it...thats just my opinion)
Silver and Gold are my two favorites...prefferably gold, because it doesnt irritate me as easy...and I love the sound the gold makes.
I have two mouthpeices, one in gold and one thats all silver:
Bach Megatone 3C (gold)
Holton MF 6 (silver)
My horn is also pure silver...so I usually have to wear gloves when I play it for long periods of time, else my hands will get dry and irritated...