Should I user Apache or Nginx & Passenger or Mongrel for my Rails application
I'd second for Passenger + Nginx. Very low memory and it's not too difficult to setup. What type of server are your deploying too? Specs? OS? I'd take that into consideration as well considering your available hardware. If you've got enough memory already, then it shouldn't be an issue whether its Passenger or Apache, just optimize and cache your app efficiently.
Two comments:You can deploy any rails app to any of your mentioned servers, so no need to decide this once and for all now.
IMO mongrel clusters are no longer worth the trouble. Go for passenger on whatever server makes you happy.
Id recommend passenger nginx, the configs are nice and tidy plus the memory footprint is really low compared to passenger apache.