Its really a subjective choice when velocity differences and accuracy is only marginal but we have used a variety of these things at the club from the infancy of Gas going legal.
In this time we pretty much proved that the early CP88 6 inch is still about the best of the lot, even when comparing the most recent models.
It utterly out revs the pathetic velocity of the PPK, CP99/Compact, also edges the SIG and the Berretta. Only using the huge length available to the S&W revolver does the velocity get matched but not nearly so matched in balance and handling when this long barrel is fitted.
Later offerings from Gamo are not so well built as Umarex.
A couple of offerings from Anics edged in power terms but are agricultural.