Honestly, thats a tough decision. I hate to put it this way, but pretty much, any era but now.
I LOVED his deep growling voice of the Core era. Hearing bootlegs of those shows gives me chills. Especially that deep growl behind "I am smellin like a rose that somebody gave me on my birthday deathbed". Crazy good.
I LOVED his smoother yet still powerful vocals of the Purple era. It rocked hard when it needed to, but was deep and melodic when it needed to be either. It was a perfect combination of his harder and softer voices.
I LOVED the Tiny Music/No. 4/Shangri Era. His voice took on that poppier sound (Lady Picture Show, Glide, Wonderful), but it still could get heavy when it needed to (I.E, Trippin on a hole, Down, Coma,). He started to overall lean more towards the poppy stuff during this time, but it didnt bother me a bit, because he was great at it.
I LOVED the Velvet revolver era. His vocals pretty much had a "All-in-one" package during this time. He had the heavy growl of his Core work in certain songs, the poppier voice of his Tiny music work during certain songs, and he was able to make it all work at once. It was like a perfect showcase for his vocals.
Now, its like his vocals are in recovery mode. They sort of have that deep growl, but its pretty much gone. That perfect harmony in his poppier songs is still sort of there, but somewhat gone as well. I hope its just because of all the touring he's done lately (Went straight from solid VR touring into Happy in Galoshes touring into STP touring and hasnt really stopped.). And i hope his pipes can recover. But if not, it seems like his vocals are definitely past their prime.