Rudisha has achieved the most over 800m, although it should be remembered that Snell didn't have World Champs to compete in during his days, and Coe & Juantorena only had a couple at the end of their careers to contest, when they were held every 4 years.
In that respect Kipketer and Rudisha have the advantage of far more opportunities to win global Champs (every 2 years outdoors, plus indoors)
No one else from modern era is in the conversation apart from Cruz. Ryun certainly isn't.
The 3 main criteria therefore should be World Records, Olympic medals and annual ranking points accrued.
World Records: -
Snell - 1 (1:44.3) - 1.4sec improvement on old record; record holder for 11 years, though equalled by Wottle & Doubell)
Juantorena - 2 (1:43.50/1:43.44) - improved old record by 0.26 sec; record holder for 3 years)
Coe - 2 (1:42.33/1:41.73) - improved old record by 1.71 secs; biggest improvement in WR since Harbig in 1939; record holder for 18 years - longest anyone has held the 800m WR for). Also 2 indoor WR.
Kipketer - 2 (1:41.24/1:41.11) + 1 = (1:41.73); improved WR by 0.62; record holder for 13 years. Also 2 indoor WR.
Rudisha - 3 (1:41.09/1:41.01/1:40.91) - in 3 instalments, has only improved the WR by 0.20 secs. Record holder for 6 years now.
In terms of number of WR's set outdoors, then the order is clearly Rudisha, Kipketer, Coe, Juantorena, Snell.
But in terms of longevity as WR holder, the order is Coe, Kipketer, Snell, Rudisha then Juantorena.
And in terms of level of improvement, then it is Coe, Snell, Kipketer, Juantorena and Rudisha at the back!
If we give 5 points for 1st down to 1 for 5th, then the totals work out as ~
Coe - 13
Kipketer - 11
Rudisha - 8
Snell - 8
Juantorena - 5
In terms of Olympic medals, then Rudisha and Snell are at the top with 2 golds apiece. Juantorena has 1 gold, Coe 2 silvers and Kipketer a silver and bronze. In order to make a comparison with the points system for World records, then I suggest 5 points for a gold, 3 for a silver and 1 for a bronze, which seems fair to me. So we have -
Rudisha -10
Snell - 10
Coe - 6
Juantorena - 5
Kipketer - 4
I would put Rudisha's 2 golds marginally above Snell's, because he front ran a WR in his first title. That in itself probably deserves a bonus of 2 or 3 points.
In terms of World ranking points, then it is quite close between the top 3:
1. Kipketer - 84 (10 points)
2. Coe - 78 (=6)
3. Rudisha - 73 (83 from Jan - 8 points)
4. Snell - 41 (=4)
5. Juantorena - 34 (=2)
This is based on 10 points for a No. 1 ranking in any given year, down to 1 point for 10th. Although it should be noted that the 2016 world rankings haven't come out yet, so it is very likely that Rudisha will jump into 2nd place on 83 points in a few weeks, yet still 1 point shy of Kipketer for another season at least.
As ranking points cover such a long period of time, an entire career in fact, then it would seem sensible to award 10 points for highest, 8 for 2nd, 6 for 3rd, 4 for 4th and 2 for 5th.
That would give ~
Kipketer- 10
Rudisha - 8
Coe - 6
Snell - 4
Juantorena - 2
If I've done my sums right then that should give a grand total of: -
1. Rudisha - 26
=2. Kipketer - 25
=2. Coe - 25
4. Snell - 22
5. Juantorena - 12
If we take into consideration Rudisha's WR while winning in London 2012, then he is a clear overall winner.
Positions 2-4 are v. close and it comes down to which category one considers more important.