This question is only possible to answer for a given set of road conditions, a single rider, and a given location and possibly only for a certain time of day at that location. If you change any of those, the answer may very well change.

You may as well say "what's the safest car?" Well, maybe it's a Volvo. Unless your criteria is that you are going over very rough terrain. Then maybe it's a Jeep. Unless you are trying to outrun a Tyrannosaurus Rex. Then maybe it's a fast sports car.

I could easily argue that my road bike is safer than my hybrid because I travel faster so I'm on the road 10 minutes less per commute, and I therefore encounter about 20% fewer cars, so I clearly have a 20% less chance of getting in an accident with a car.