I would have to say the Vulture simply because it is so cheap it is a throw-away.
So you can do as much PvP in it as you want without the result making you bankrupt or anything. And of course the more you try the better you get.
Cheap in the sense that the Vulture can earn you its rebuy back in just a short session of grind.
And being so cheap it is still a formidable combat ship.
On the other hand, if you lose a Phyton or FDL, you are stuck on the grind-pain for how much time?
As Stalin said, quantity is an outstanding form of quality.