Worst I've ever flown was the Citabria. Its fine in the front seat when cold outside. But the only warm air outlet is right under the panel. The plane is so drafty though, that while the front seater will be toasty warm, the guy in the back will be, quite literally, still frozen.
I can imagine though that pales in comparison to, say a Waco or Stearman open cockpit biplane. Or an ultralight.
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