Here are my Top 5.
Peugeot 905B. I had a small book for kids that showed how cars worked and had several cut-away car drawings, one of the most intriguing being of the 1993 Le Mans-winning 905B. Watching footage of the car at racing pace and hearing the sound it made later only solidified how special of a car it was.
Williams FW15-C. It was the first F1 car that caught my eye and getting up early in the morning to watch live F1 races with my dad the year Alain Prost won his last driver's title made this car all the more special, especially as it was the last year of all sorts of driver aids allowed in Formula 1.
Penske PC23-500i. 1994 was the year Team Penske won 12 of 16 races in CART and the first year I really followed Indycar racing outside of The Indianapolis 500. The '95 500 was the first I can remember watching all the way through to see if the one-off, 1,000 HP Mercedes-Branded Pushrod Engines could last the distance. Reading through the book "Beast" by Jade Gurss really brought home just how big of an achievement the whole Penske organization achieved that year, especially in light of them failing to qualify for the following year's race...
Mitsubishi Lancer EVO VI Rally Car. A combination of Gran Turismo 2, buying imported Autosport magazines from a Borders bookstore that was around 3 hours away from my house whenever I could, getting satellite TV and experiencing the wonders of Speedvision's World Rally Championship coverage hooked me on the one of the great eras of the sport, with Tommi Makkinen & Mitsubishi at the top of their games and winning rallies left & right with a Group A car against the earliest purpose-built WRC cars.
Panoz LMP1 Roadster S. The American Le Mans Series was in it's infancy and the battle between the BMW V12 LMR and the thundering Panoz was a great sight to behold when me and my family took a trip to watch the ALMS race at Laguna Seca. The fact that the Panoz was still fairly competitive even in the face of the all-conquering Audi R8s when the design itself was based on the old "Batmobile" GTR-1 made it a favorite.