Bare Minerals is my standard base most of the time, but on hot days, it doesn't stay put unless I have the lightest of light layers (not always possible as my skin breaks out).
Almost Makeup was so, so good as a summer base - enough coverage, nice finish, and it could contend with your face sweating on 40 degree Northern Line carriages, and then resetting again without streakiness. I miss it still after all these years!
Has anything come along recently that works well in hot conditions?