What Should I Replace Forefront TMG Firewall With?

Microsoft formally announced the end of Forefront TMG 2010 firewall late last year.  Whilst mainstream support will be available until 2015, many IT decision makers are already asking themselves “What should I replace my Forefront TMG firewall with?” Since Microsoft has elected to leave this space entirely, you’ll have to replace it with a solution from another vendor. But which one?