It depends a lot on personal preference. I don't know a lot about Panasonic, but Canon is famous for making still cameras. As a result, their cameras have great lenses, which improves picture quality and, often, a Canon will have a 12x or 14x optical zoom where an equivilent Pan or Sony will only have a 10x optical.
Sony, on the other hand, manufacures electronics. They're cameras are a bit more rugged, they don't have that "plastic" feel that the Canons and Pans do. This is why a lot of rental places and film schools use the PD150s, VX2000s, and TRV900/950s over the Canon or Panasonic. They're more durable (I think their cases are titanium or some alloy thereof), and their knobs are less likely to break off in your hands.
I've handled both the Sony PD150 and the TRV900, and they're both quite easy to use, the PD150 a bit more so because its a little higher-end than the TRV. I haven't used the Canon or Pan at all, so I can't comment on that. Though, in my search for a camcorder thus far, I've found the Sony hand down is easier to oprate than the Panasonic or Canon - but I haven't tested very many.
The best thing to do imo is to check the internet. Just type in the camera you're looking at and you should get a good deal of reviews, and comparisons with other cameras.
Good luck!