I got my "grail" coin last year. Empress orbiana. You can read the story of it at http://www.cointalk.com/t211137/ My next would be an affordable yet decent Aquilia Severa. Orbiana (225 - 227 A.D.) AR Denarius O: SALL BARBIA ORBIANA AVG, draped bust right. R: CONCORDIA AVGG,Concordia seated left holding double cornucopia and patera. 19mm 3.8g RIC IV 319, RSC III 1, BMCRE VI 287, SRCV II 8191
Like you, a nice Santiago mint 8 reales was also my 'grail' coin. I finally got this Carlos III from a Superior Galleries auction back in 1990. Back then there were less than five known of this date. Nowadays I would say there are close to 10 known. This is probably one of the better ones.
My 'Grail Coins': Carthage, AU 20-shekels, Head of Tanit/ Horse with Head Reverted, 12-13 grams Carthage, EL 30% 12-shekels, Head of Tanit/ Horse Prancing, Palm Tree in background, 22-23 grams Carthage, AR 5-shekels, Head of Tanit/Pegasus, 31-40 grams Carthage, AE 15-shekels, Head of Tanit/Horse, 96-121 grams I love Carthage... And Large coins... Hence, my 'Grail Coins' are both!