We use Freshdesk. We need to pay for 3 instead of 4 of us, as first agent is free. For a SAAS startup that is just getting started this fits our needs perfectly - support tickets that are captured via mail and listens to conversations on twitter, facebook and converts into tickets. Saves plenty of time. All the other features you mentioned are all covered - like knowledge base, feature requests etc.,

Disclosure: We are friends of Freshdesk team - Zoho alumni network. :)