Finpension would be the cheapest, though I don’t understand the apparent sudden lust for the S&P500 we are seeing on this board. US funds have one of the best tax treatment possible for Swiss investors: low dividends and fully recoverable witholding taxes if held through a US domiciled fund (given your regular taxes outweigh the tax savings and you fill the right paperwork). They are best held outside of tax advantaged accounts (that is, there are other funds that would benefit more from it).
Why US stocks specifically?