Been playing with this one for a few months now. RT below hits the nail on the head; the tequila will dictate the outcome. Use Siete Leguas, Lalo or an equivalent. Try a Reposado. I have found the Italian Martini & Rossi to be the best match (over Dolin, my fav gin vermouth) but dial it down to 3:1. Bitters optional, but absolutely worth playing with. Orange, lemon or grapefruit are all good choices. This is absolutely the best summertime, pool-side Martini I've ever had (I've had a few).