1.Oberleutnant Günther Viezenz-a company commander in Gren.Rgt. 7 with 21 tanks destroyed.
Apparently he received KC for »only« 7 destroyed tanks and also told some collectors that this was also his total amount. You can clearly see on this picture that he's wearing only 7 Panzervernichtungsabzeichen and all of them are in Silver (though they don't all look the same because of the sunlight).
2.Leutnant Friederich Anding-18 kills
Friederich received his KC for the destruction of 6 tanks and 5 armored vehicles (so says his Verleihungsvorschlag zum Ritterkreuz), as adjutant of the Pz.Jg.Abt. Großdeutschland (commander of the battalion was Maj. Walle) on 8 May 1945. This action took place in northern Germany (more specifically in Stadensen) on 14-15 April. The battalion was attacked by a large number of enemy tanks and armored vehicles. Major Walle (9 destroyed tanks), Leutnant Anding and Obergefreiter Stützle (7 destroyed tanks) received KCs for their actions.
Now here's what Andreas Düfel (the webmaster of das-ritterkreuz website, he also lives near the town Stadensen, where this action took place and talked to several witnesses) says about this particualar incident:
»..The town was almost completly destroyed by the tank battle. A documentiation about this fightings still exist. It's interested that the town residents doubt whether there were really18-22 British tanks destroyed. The wrecks were predominantly armored vehicles (not tanks) and quite a few of them were also German. War confusions of the last days of the war could have quite led to a false evaluation of the actually destroyed tanks..«
Also an interesting point from Manfred Dörr (the author of the book Die Träger der Nahkampfspange in Gold):
»You mustn't forget that when two soldiers attack and destroy the same tank, both of them can claim they were the one's who destroyed it. That's why it's theoretically possible that Walle, Andig and Stützle together destroyed 6 tanks but they all received 6 stripes (Tank destruction badge in Silver) each.«
Furthermore here's Anding's picture, you can quite clearly see that he has »only« 6 silver stripes.

3. SS-Untersturmführer Frithjof-Elmo Porsch credited with as much as 24 destroyed tanks and a holder of DKIG, NKIG, Ritterkreuz and other similar awards.
There is quite an interesting story about Porsch and his companions from SS-Panzerjagdkompanie »Dora II« in several books (Lucas and Krätschmer), but according to his personal file in the Bundesarchiv he received only EK2 and EK1. From what I gather he was (and still) is a notoriuos lier.
Next three were considered for a long time KC-holders, but apparently they're not and since all three received KC for their tank busting exploits their total tank kill numbers are doubtful to say the least.
4. Standartenjunker Willi Fey-14 kills
First shock: he apparently wasn't even a Knight's Cross holder so the number is very much questionable.

5.Ustuf. Wilhelm Weber-the famous commander of Nahkampfschule »Charlemagne« (another non-KC holder, but credited with 13 kills)
6.Ustuf. Karlheinz Giesler (non-KC holder, credited with 11 kills)
This would leave as with Ustuf. Adolf Peichl as the one with the highest number of confirmed tank kills (11) and maybe Oberleutnant der Schutzpolizei Heinz Heuer with 13 (not yet confirmed).
You'll probably also notice that a few other KC holders in fact also didn't receive the award: PAUL EGGER (the panzer ace), JAKOB LOBMEYER (considered for some time even as an Oak Leaves holder) and GUSTAV-PETER REBER (a very interesting story about his award ceremony)



A new revised list:
Peichl 11
Egghardt 9-10
Walle 9 (??)
Mangold 8
Vaulot 8 (??)
Fink 7
Stützle 7 (??)
Viezenz 7
Anding 6 (??)
Apollot 6 (??)
Kiesgen 5
Nietert 5
Voigt 5
Ahrens 4
Kaiser 4
Konopka 4
Kuhn 4
Leykauf 4
Spranz 4
Bäke 3
Jahn 3
von Rohr 3
Vogt 3
Ehm 2
Lichte 2
Lorenz 2
Schwerdfeger 2
Of course this list is far from being complete so every contribution is welcome also any thoughts on this post would be great. And please visit also das-ritterkreuz Forum for further details.
Regards
Mark