Rza, just blaze, kanye or Dilla?
I think some of it depends on whether you're talking who is the best at sampling soul, or who is the best at giving you a certain
feeling when you hear the music. Dilla sampled a Janet Jackson pop record and it had a lot of soul to it.
Of all time? I definitely would have to go outside of your list and go with Pete Rock. The horns, the bass lines...Pete is THE soul producer IMO.
Out of those 4 that you named? That's tricky. Dilla sampled from so many sources and had so many styles and eras. If you took Dilla's best soul beats they'd probably crush anyone on the list, but Just Blaze beasted a ton of soul records also. I definitely think Kanye did it the easiest. No knock, but he took a lot of real obvious loops or sped up vocal samples, and threw his clap/clack drums on them and then the bongo/congo drums. Still had a lot of fire.
I always like to judge based on albums instead of random songs, so RZA would be my #1 out of your list. Ironman and a lot of Only Built 4 Cuban Linx is enough right there to probably put him at the top spot. Just Blaze is the sleeper on your list, for sure.
Currently Madlib, 9th Wonder, and Alchemist are probably my 3 favorite producers that have a heavy soul influence. Madlib and ALC both sample a lot outside of the soul genre right now, but when they do decide to chop or loop soul I think they do it better than anyone in 2014. 9th mostly sticks to soul records as a source of inspiration, so he's pretty much always on the list.