You said that Genius and Superhuman Chryssalids are guaranteed to hit and thus zombify, but the info on UFOpaedia says the zombification always happens when they stop attacking a unit, whether or not they hit it. Which is correct?
On-topic, Hovercars for the first week - you're generally limited by the supply of Bolter lasers more than anything else, and Hovercars come with one attached so they only need one more (though I suppose you could buy the Hovercar and take the laser for a Hoverbike). Also, Hovercars won't die from a single Light Disruptor hit, while Hoverbikes can.
For second week onwards, I'm thinking a mix of cars and bikes is probably best. Cars are way better once you get shields, but depending on your research order that might take a while.
Generally, I find that due to the long lead time of the Advanced Workshop, I generally have Retaliator tech done or almost done by the time I can actually build craft; the Explorer is certainly a great ship, but if you have to choose between that or a Retaliator there's no contest. Annihilators... eh, I build them, but usually just because I can't think of anything else to do with my money; by the time you get them, you'll have a few Retaliators in the sky and won't actually need them. The firepower isn't even that much better (though the shielding is).