The Continental Zippy 1 a favorite tire amongst many vintage-turned-modern scooterists, a great performing scooter tire, yielding somewhat better longevity than the Pirelli's on the ET series. Available at nearly everywhere scooter tires are sold, the continental Zippy 1 is a known entity, a good choice for the ET series scooters.
See also: LX150 Rear Tire Change (Video Tutorial)
Vespa decided on the Sava MC20 Monsum tire for the GTS, GT60, and GTV scooters, but seems to be universally disliked amongst the MV community. That tire choice by Vespa arguably may have in part been due to its ability to mask or minimize "head-shake" or wobble, a somewhat common anomaly on the GTS/GTV, and to a lesser extent on preceding models. The Sava does not handle or wear well. In particular Sava tires, due to their center groove, have particularly bad "tracking" behavior on rain grooved roads. Many owners replace the Sava tires prematurely in lieu of other brands, with Continental Zippy 1's again being a popular choice, as are the Pirelli GTS series and the Heidenau K61.
For the 250 Super and 300 GTS, GTV, and Super models, Vespa has been delivering them with Pirelli GTS23 F and GTS24 R tires. This choice of tire may be a contributing factor in these models not having the head shake (wobble).