I always have a hard time with questions like those because I find it difficult to compare players from totally different eras of the sport.
Comparing the Poltera brothers (40's & 50's) to the likes of F.Hollenstein and Jaks and then further on to modern players like Hischier and Josi is almost impossible to me, the game has changed so much. But since the Swiss league is producing NHL players on a regular basis just for a few years now I'd say the level is definitely higher now than 20-30 years ago when Felix Hollenstein and Peter Jaks careers we're on their height. Otherwise, Walter Brown, former president of the Boston Bruins, said back then in the 40's that the Poltera-Line could easily play in the NHL, shame they never did.
Long story short I'd say that it is pretty impossible to make a good statement on the best player ever, but in my opinion it's probably Roman Josi right now, he has all it takes to be considered an absolute world class defenseman and is probably the face of Swiss ice hockey at the moment.