I can't stand t-shirts in hot, humid weather. They stick too much, especially along my upper arms, and the closed neck keeps too much body heat in. For me, it's a cotton button-down, usually long sleeved. (I roll the sleeves to my elbows or higher 90% of the time, but long sleeves at least give the option of wearing them down.) I'll keep at least the top two buttons undone for airflow. A light weight cotton is preferred (Indian madras is fantastic if you can find it, but if you're paying retail it'll hurt), though even a relatively hefty Oxford cloth is usually fine.
For bottoms I'll typically wear canvas pants from the farm store. Something lighter weight would be nice, but I make use of the pockets on the carpenter-style pants and those aren't, to my knowledge, readily available on lighter weight pants. I'm also quite hard on pants, so a durable fabric is a plus. Occasionally I'll wear shorts, but only if I know I'm only going to be working in the garden for the time being.
Silly as it may be, I prefer the look of a tucked in shirt, but they usually come untucked rather quickly, and untucked is certainly cooler.
Except for during morning chores, when I just throw on muck boots, my footwear consists of wool socks and leather boots. I'm surprised how many people think cotton would be cooler, but wool wicks moisture, keeping one's feet dry and comfortable, and by whatever mystery of the universe doesn't make for hot feet. I do tend to put on fresh socks midday for comfort.