I'm sorry; as great as he was I just don't think Snell is really a contender for the title of greatest 800m runner of all time. Short career, without a tremendous depth of competition (certainly compared to the incredibly deep eras of Coe, Cruz, and later Kipketer). Roger Moens is the only serious competition I can think of Snell encountering. Obviously Snell was very strong runner, but I doubt he would have had the wheels to live with the event's later champions.
Also, I have trouble placing Cruz ahead of Coe. For the title GOAT, it is important to consider championship record, times, competition, records, longevity and consistency. Here's how Cruz and Coe match up over 800m:
Champion Medals
Cruz - 1 OG, 1 OS, 1 WC bronze
Coe - 2 OS, 1 ECG, 1 ECS, 1 ECB
World Records
Cruz - None
Coe - 2 (1:42.33 in 1979 and 1:41.73 in 1981); Coe held the 800m WR for a total of 18 years, over two incredibly deep running eras.
Times
Cruz - 6 sub-1:43 performances
Coe - 2 sub-1:43 performances (both WR's)
Longevity (Years from first major championship medal to last)
Cruz - 4
Coe - 9
It is certainly a tough call; Cruz takes Coe in Olympic performances and number of fast times, but Coe was the world's fastest 800m runner for 18 years, was consistently competitive for much longer, and had his own impressive championship medal haul (I include Europeans because Cruz contended in the Pan-American games, even winning a 1500m gold in '87). Ultimately, if you were to match up Coe in his prime ('81) against Cruz in his prime, I think Coe would win 9 times out of 10.
1. Kipketer (there should really be no debate here)
2. Coe
3. Cruz
4. Snell