I find breaststroke the least demanding (i.e. it leaves me the least exhausted), but I find freestyle easier. The actual motion of backstroke is easy, but I can't swim in a straight line if I can't look ahead while swimming, so I tend to avoid it as it's difficult to prevent myself from weaving from side to side in the lane or bumping into other people. Butterfly I find the most physically demanding.

I also learnt to do sidestroke and survival backstroke — Australian swimming lessons for children are big on water safety, and we were taught that sidestroke, breastroke or survival backstroke were the best strokes to do if we needed to conserve energy and swim for a very long time. Swimming lessons in Australia also teach freestyle as the default — everything is building up to getting you to the point that you can swim a length of freestyle in a 50m pool without difficulty, and you only really learn the other strokes after you've already competently learnt freestyle. For this reason, I've always had much more practice swimming freestyle, and tend to only use that stroke when swimming laps.

I realise I haven't really answered your question. For me personally I think backstroke is the hardest, but butterfly is the most physically demanding.