One way to decide this, is looking at WCh being able to beat former, current or future (other) WCh's as BLACK in regular games. Here is the list:
Euwe: 1
Kramnik: 1
Steinitz: 1 or 2 (depending on Anderssen being WCh).
Alekhine: 2
Karpov: 2
Smyslov_Fan considered Spassky the 2nd weakest WCh. But he holds the record, having beaten Smyslov (ouch, that hurts), Tal, Petrosjan, Fischer, Karpov and Kasparov having Black.
Kortsjnoj, Larsen and Geller hold the record for non-WCh's: they all have beaten 7 WCh having Black.
Before WWII it was my beloved Spielmann, who managed to beat Alekhine in Karlsbad 1923, Capablanca in Bad Kissingen 1928 and Euwe 2 times having Black.
Like all other arguments this probably makes absolutely no sense.
Another meaningless way to look at this question then: which WCh's played the fewest WCh-matches?
Fischer: 1
Capablanca: 2
Tal: 2
Kramnik: 2
Euwe: 3 (1948 tournament included)
Spassky: 3 (lost 1966 match).
Petrosjan: 3
If possible, my vote would have gone to Ruy Lopez.
BTW: what do you mean with OFFICIAL WCh? The first one recognized by FIDE was Botvinnik!
There are some arguments to consider Anderssen, Morphy and even Staunton having been WCh too.