Besides many of the kits already mentioned, I'm also impressed with:
1: Revell's and AMT's parts packs because of their staying power, quality for the time they were tooled (early 1960s) and relevance to today's hot-rod builders.
2) The old Mickey Thompson Challenger and Attempt, and Ivo's Showboat. Very complex, well-detailed kits, especially for when they were tooled, and impressively rendered subject matter.
3) Revell's model A Ford series, all of them. Still a great parts source, and still relevant to today's hot-rod scene.
4) Accurate Miniatures Corvettes. Beautifully tooled and detailed.
5) Accurate Miniatures McLarens too. Same reasons.
6) AMT's original AlaKart / '29 Ford roadster double kit. Lotsa parts, good accuracy, and again, still relevant.
7) Revell / Monogram 1/8 scale '32 Ford. It's just outstanding, and can make a show-stopping model.
8) Fujimi's Porsche 356 and 911 Enthusiast kits. Much much detail, possible to build the really landmark and important cars from this manufacturer.