There's 2 main styles of R&B I think - the 'traditional' sounding R&B that was most popular in England in the 60s - John Lee Hooker, Sonny Boy Williamson, Howlin Wolf etc. and the type that took the R&B rhythms & chords but added big city / pop production values - and I guess in the end became soul.
I got to be honest and say I prefer the more produced big city sound made with arrangers, bandleaders & bigger bands. I often find the traditional r&b sound a bit formularic and samey but I really like examples of the more traditional sound too, like Howlin Wolf 'spoonful', Sonny Boy williamson 'help me', John Lee Hooker 'Money' etc.
There's no line where one style stops and the other starts and loads of great artists and records cross over into both styles. Tommy Tucker, for example, released the very nice but certainly more pop/soul flavoured 'Oh What A Feelin' in the UK the same year as the more traditional sounding 'Long Tall Shorty'.
I think any claims that one is more 'authentic' than the other is bogus, I dont think authenticity is relevent to pop music. I actually think that the Englishman's tendency towards retro-ism & purism kept the traditional blues / R&B sound popular in this country long after American R&B artists really had moved on. But to an extent I think the Americans probably fell into two camps - those who embraced the pop/soul movement pioneered by labels like tamla & King/federal and those who stuck to the authentic sound. A lot of the old blues men never even added horns or anything to their sound and it was pretty much the same in 1970 as it was in 1950. I dont really see the point of that.
R&B classics would most likely be of the more traditional style, as that has been popular and played longer, the more produced stuff only really becoming popular in the last 10 years.
Classics of the latter style would be stuff like Mike Pedicin 'burnt toast & black coffee', Joanne Henderson, Five Royales, Joe Tex 'I wanna be free', Ernie Washington ' lonesome Shack', Cookie Jackson ' i got to know', Charles Sheffield etc etc. But these have far from hit the mainstream in the way stuff like John Lee Hookers 'boom boom' or Muddy Waters 'I'm a man' did.
The soul scene takes more to the better produced, and often later, r&b as it shares more production values with 60s soul.
sorry for the ramble