Blues Image, "Ride Captain Ride" For some reason, that really struck me as a teenager.
Actually, I'd put several in front of that, but they've been mentioned -- Green Eyed Lady, 96 Tears & Spirit in the Sky at the forefront.
By the way, the Mountain Climbing album is an excellent group of songs (e.g., Theme for an Imaginary Western written by Jack Bruce) so even though they only had one big hit (Roll Over Beethoven was a smaller one perhaps), they had one of the 50 best albums made in rock, IMO.
Also, I heard an interview with the guy who wrote "This Diamond Ring" (can't remember the name) and his vision for that was very bluesy/R&B. To this day, he hates the pop rendition. He has a version of his own which is very good, but I love the pop one.