Great topic!
As a kid riding in the back seat of my Dad's '72 Dodge Dart Swinger, I had a hard time differentiating the similarly sloped nose and big round headlights of Alfa Spiders from Porsche 911s in oncoming traffic. They both grabbed my attention instantly; even my 7 year old brain knew these cars were special. That was my first exposure to Alfa Spiders, and Porsches, for that matter.
Then, in 1990, back when I was proudly driving a 1980 Triumph Spitfire, I sat in an S4 Spider at the 1990 Chicago Auto Show. It felt great, an order of magnitude better than my Spitfire, and decided then that I'd someday own one. In 2000, I was lucky to get an air cooled Porsche 911 while they were still affordable, but I still had an Alfa itch to scratch. So in 2002, I made an impulse purchase on eBay for a '69 Alfa Spider that was restored and upgraded (hot 2L engine, suspension upgrades). Took a chance buying a car sight unseen, but it turned out to be a great car that I still have. Bulletproof, in fact, aside from very slowly progressing rust bubbles in the rockers. It's been my daily driver in the few acceptable roadster driving months of the Pacific Northwest where I live. For almost twenty years now. Dang.
Flashing back to 2003, I had just started racing Spec Miata as a way to keep from further thrashing my Porsche at PCA track days. Within a couple years, I was losing interest in it as I couldn't ever get past mid-pack, never had a passion for the car, and eventually learned that you couldn't be competitive unless you were creative with the rules interpretation. Granted, I was running a junkyard engine, and was loathe to tinker on and "develop" the car beyond the spirit of the rules. When I sold it in '06, I planned on racing vintage in a car I actually loved. Old Porsches were (and still are, more so) prohibitively expensive, so I looked for an Alfa to race. I stumbled into a trashed, rusty roller of a '69 Spider with a seized engine for about $2500, and many thousands of dollars later, built it into a race car. Completed around 2010, and has a long thread of its build on AlfaBB. Been racing it with a passion missing from the former Miata, and have enjoyed reasonable success including multiple podiums.
So my pick of best Alfa Spider is the '66-'69 roundtail, "Duetto." I feel like the roundtail Spiders (boattail is really only accurate for old Auburns and Rivieras) are the cheapest exotics you can own; like 3/4 scale Ferraris, and they really look out of this world, especially when parked amongst modern cars. Here's a pic of my two Spiders, street and race: