There's no perfect seat for every situation. With rifle main considerations are:
- how big sector you need to cover
- how much stability you need for shooting
- kind of related, how much noise/movement can you make w/o spooking the quarry
- carry, for how long and do you need to set up once or every once in a while
- and how comfortable you're able to get, taking the above in consideration
In addition to other replys so far, I'd highlight the ability to support your shooting hand elbow on either arm rest or your thigh, i.e. chair cannot be too high. And have a look on different carp chairs like suggested. One thing not mentioned yet is "director's chair". They're quite straight forward to set up and carry (if you can spare a hand), somewhat easier to cover a sector than folding camping chairs. But they do require flat ground. On that regard, 3 leg chairs fare usually better than 4 legged ones.