It seems orange surf unlimited for 35 / month is best currently out there..
Unless you need not more than 1GB then:
1GB - lebara -15chf
1GB - yallo - 15chf
1GB - aldi mobile - 15CHF
1GB - orange - 15 CHF
1GB - OK mobile - 14CHF
1GB - sunrise - 15CHF
So basically 1GB -> 15 chf on pretty much most / all networks... sometimes however networks like lebara have promotions on top-ups meaning that for example for 100chf topup online you'd get 250 credit (that was the case at some stage).
EDIT: Ah I got a mixed up, lyca seems the way to go they have:
1GB - 6CHF / month - details here: http://www.lycamobile.ch/en/great-sa...n-data-bundles
also they give you MORE money they you pay in so effectively package is even cheaper: http://www.lycamobile.ch/en/free-cre...-online-top-up