I have been a blues fan for many years. Really there are two forms of blues, pre-war acoustic (country) blues and post-war electric (urban) blues. Country blues are mostly solo artists or duos - guitar and vocals, sometimes a harmonica or other instrument. Urban are mostly small bands - electric guitars, bass, drums, harmonica, horns plus vocals. Most people expect to hear urban blues when they go to a blues concert.

I agree with Earl in his choice of recordings, for country blues. I would add "Canned Heat Blues - Masters of the Delta Blues" RCA-Bluebird-BMG 61047-2 which features Furry Lewis, Tommy Johnson, and Ishman Bracey. Also "Masters of the Delta Blues (The Friends of Charlie Patton)", Yazoo 2002, is good. Yazoo has re-released a lot of country blues from 78 RPM records, with great liner notes explaining them. But be prepared for a lot of hiss, pops, and noise.

For another ragtime flavor, try Big Bill Broonzy. Yazoo has one called "The Young Big Bill Broonzy" that I enjoy.

For a real gentle country blues, Mississippi John Hurt can't be beat. He did a live double album set in the mid-60's (when he had been rediscovered) that's really good.

For urban blues, I recommend the "Living Chicago Blues" series - some great stuff by lesser known artists.

Muddy Waters and Elmore James were great transition artists between country and urban - any of their stuff is worth listening to - but Muddy had a more pleasant voice.

Jimmy Reed played a very basic and effective style of blues with a small group sound. "Big Boss Man", "Goin' to New York", and my favorite "Insurance Blues" are songs to look for.

Buddy Guy (often with Junior Wells) is always great.

Of course B.B. King is one of the standard beareres of the blues. I like his early and late stuff but I try to avoid most of what he did in the 70's - 80's when he seemed to forget the blues and appeal to a wider audience with soul/pop. I was disappointed when I saw him in 1980 or 81 and he had almost an entire orchestra (and synthesizers) backing him up.