Subjective or not, here I come..
Interesting to read that there are folks who dislike Elvis' version of The First Time Ever I Saw Your Face. I just like the sound of Elvis' voice better than Flack's. Although her version is very good Elvis has got something special, maybe it's his experience as an entertainer. I think that Elvis' versions of this song got better and better over time.
I don't think Green Green Grass Of Home is better with Tom Jones. Tom is a cool and nice guy, but his sound is so ' Frank Sinatra-like' in a way that he seems to sing from his throat a tad too much(and not enough from his midriff). Elvis has got more emotion and passion than Tom does and in my opinion has got a better balance between hard and soft as well.
Willie Nelson is also a very loveable character who likes to smoke marihuana responsibly, but his voice is nowhere nearly as good as Elvis'. I definitely prefer Elvis' versions of Blue Eyes Crying In The Rain and Always On My Mind above Willie's. I guess a factor was that Elvis was having trouble with his marriage at the time of singing Always On My Mind, so it fitted his mood rather well.
I must say that Jim Reeves did have a carefully developed beautiful singing voice and I like his version of He'll Have To Go. I'd say that Jim's version of the song would definitely give Elvis a run for his money, so to speak, but I like both of them. Elvis does succeed in making it his version, not too close to the original, so it is innovating enough.