Not having actually done fire spotting, but some other aerial observation and mustering work in my youth, I kinda like tandem high wing machines like the Super Cub. Enough power when solo, good visibility from either side of the cockpit, open doors to keep cool and a low stall speed so if it quits at least you arrive at the accident slowly. Someone mentioned the C172. Compared with the Cub, for low level work at min speeds it would not be my first choice and if it was inevitable, I'd rather crash in a Cub anyday.