i made a pair of machetes (if you're interested you can see photos in the gallery on my website) out of 1/8" S7. they only had blades 12" long, so they are very stiff. i had paul bos do the heat treat for me.
they chop pretty well for their weight, but they are still pretty heavy. i have a couple more on the bench now from some 3/32" S7 that i'm hoping will be a little more flexible. i looked at my machete and saw that its about 1/16" thick shockingly enough...very thin and flexible. i want mine to be a bit heavier but i want some flex in the blade.
i think that 1/8" is a good thickness for a heavy machete.
but your question was on steel...i like S7 since its highly shock resistant and its also held a good edge in my testing. i think that if you treat it right most carbon and tool steels will work for you. by the way i had paul bos do my blades to 57 RcH and they have held up great.
