Smashbox photo finish works well in the winter, or for drier skin types. For warmer months, or oilier skin, I find that MAC Prep+Prime works best. It controls oil better IMO.
I think that it's fine to use liquid foundation in hot weather, but just as long as you use a good primer and top it off with Blot powder. To me, the pressed works just as well as the loose and is less messy.
Of course, if it still melts, it's best to use a primer and follow it with a powder foundation. Studio Fix is my favorite and wears super long.