oh I don't know the best brand. hot hands are certainly common. I have found that some older units go bad, so it's good not to overbuy & buy fresh units each winter. but that doesn't mean older units don't work. used some yesterday from last year, worked perfectly. when I was commuting, I learned that if you ride in to work, removed them & bag them in a plastic sandwich bags w/ no air, they will stop the chemical process, then you can use them on your return ride back home, later in the day

dunno about why I started using them on top. some ideas, seemed logical. up there they won't get scrunched up from the toe/foot movement. the top is the coldest area due to wind & precipitation. the top has room for an air pocket in the socks