I know that of the chillies you can get in supermarkets, the birdeye are the hottest. But I don't know if that is a general rule of thumb. ....I don't know what the size is of the legendary Dorset naga.....
Supposedly the membrane and seeds hold a good deal of the heat.
Generally speaking, I think the smaller chillies seem to be the hottest, but the supermarket ones don't seem to "bite" all that much. Go to an Asian outlet, or market where you can search for some really good ones.
Seriously - pick your own??? Mind you, I think once they crop - they crop! Bet they can't get rid of them fast enough, because they'd grow well in the heat, but shrivel quickly I'd imagine?