[charlesk]

Having run most of the platforms mentioned above (not run PR) on the toughness front I would suggest that the XB2 is probably the strongest.
The RB6 was also very strong (can't comment on the RB7 but quite a lot of that car is the same as the 6 so would expect it to be still good).
The B6 platforms are also tough but in my experience not to the level of the above (currently run a 6.1, which is great to drive, but certainly not as tough as the xb2 I had before).
Parts support for any of them is good, associated and x-ray probably a little more common, but Kyosho is certainly also good.
Good thing on the X-ray is that you can buy the part you've broken on it's own (ie 1 arm rather than a pair etc).
Any of the cars mentioned will be a good choice, would suggest to go with whichever one is most common where you run...