Oh man, this is way too hard!
I think his voice was at its most versatile (as far as depth, tenderness, rawness, volume, warmth, range and control goes) pretty much from the '68 Comeback through to perhaps 1971.
Before and after that, in my opinion, some of the ingredients was always missing. So when he was young(er) he missed a bit in depth and thus range, but excelled in enthusiasm.
In 1973, primarily, I think his voice lacked warmth often (like in the Aloha days his voice sometimes had an almost metallic, nasal, edge to it, sharper than his usual smooth tones) and maybe even a bit of control, resulting in some wavering notes.
Later when his health started interfering with his vocal abilities it seems he sometimes was forcing his range by yelling, just to make it seem he was still as powerful as in his healthy days.
This is all just MY perception, how his vocals during the years come across to me, okay, so don't hang me up on some high branch of a big ole tree
I still have vocal favorites in each phase of his career, though, because often "weaknesses" or "flaws" can fit a certain song perfectly, mood wise, which is why Elvis himself often did not do retakes of songs with mistakes in them, because even the flawed versions "felt right".
So, taking the above into consideration these are the first songs that come to mind, in no particular order:
Trying to Get to You ('68 Comeback, vocal range, versatility and strength)
If I Can Dream ('68 Comeback TV version, vocal range, versatility and strength)
American Trilogy (Aloha, probably the best sung song during this concert)
In My Way (Wild in the Country. tenderness, warmth)
You've Lost that Loving Feeling (TTWII, energy, range, rawness)