The R22 is best by virtue of being the cheapest and most available for training, if you're not too heavy for it. It takes a lot of hours in practice to be ready for PPL (more than the minimum) so the money really adds up. The Cabri G2 is roughly twice as expensive as the R22 and thus so are the training costs. In fact it's very close to the price of a base R44 Cadet.
The G2 is for sure safer than the R22/R44 or Bell 206/505 just by virtue of having a fully articulated rotor instead of semi-rigid, and a fenestron, but in practice accidents almost always have nothing to do with the rotor, especially when flown with a competent CFI.