Comparis is very good as is Immoscout.ch and homegate.ch. Comparis apparently "pulls" listings from the 2 mentioned websites but we found a great apartment in Zurich, listed on Immoscout which, for whatever reason, was not listed on Comparis.
Be aware that most apartments are listed for rent in April and October and to a lesser extent, June. We arrived in mid-April not knowing this and it made our search tricky to say the least (most of the "good" apartments were already rented)

. But we got there in the end and have stayed in our current flat for 4+ years.
It is not easy at all to find large kitchens -notwithstanding eat-in kitchens in Zurich - especially in serviced accommodations. Much depends on one's budget!
Good luck with your search...