A troll car is one that dominates its race class effectively from exploiting a certain advantage. Whether that advantage was an entirely new concept or just one that had been improved on, the troll car used it so well that it won most, if not all, of the races it entered. Most troll cars were so good they caused their governing motorsports body to change the rules just to specifically outlaw that car.
Chapparal 2J- It had the sucker fans. At that point in racing aerodynamics were a largely untapped source of grip. As everyone knows, the faster you go the more downforce you create and the better you stick to the track. The 2J used its fans to create massive amounts of downforce at lower speeds, thus creating significantly more downforce than any other car on the track at low speeds and allowing to corner better. It wiped the floor with the competition. Hard.
Lancia Stratos- This was the first dominant car of world rally. The first car ever designed specifically for rally. It owned Group 4 for 3 years and was competitive for several more.
Lancia Delta HF Integrale- This thing trolled world rally stages for years with is Torsen differential and advanced braking system. It still holds pretty much every WRC record for amount of stages and championships won.
Audi Quattro- In 1979 FISA (FIA now) legalized 4WD systems in rally. Most manufacturers thought the systems at the time were too expensive and heavy to have any practical application. In 1980 Audi showed the world that if you wanted to be competitive in rally, you better have 4WD. Although for the first two seasons its results were inconsistent due to reliability problems, the races that it finished were won by enormous margins. However, in the newly created Group B, it would become one of the major players and then go on to claim many victories in Rallycross GT.
Chevy Corvette C5-R- There is no replacement for displacement. And never did that famous quote of Enzo Ferrari ring more true than when the 7.0L Corvette C5-R manhandled Le Mans endurance racing and ALMS (in 2001 Dale Earnhardt and his son learned how to turn right in this car). It managed wins (even some 1-2 and 1-2-3 finishes) at 24 Hours of Daytona, 12 Hours of Sebring, and 24 Hours of Le Mans as well as a dominance of ALMS. In 2002, the Corvette C5-R only lost one ALMS race.
Ford GT40- Ford attempted to buy Ferrari in the early 1960s. When that failed, they decided to send a big FU to Ferrari by beating them at their own game: European endurance racing. Originally powered by a 4.7L V8 but soon upgraded to a 7.0L V8, the car won the 24 Hours of Le Mans 4 times in a row. The FIA placed engine displacement rules to keep Ford's 427 engine out.
Nissan Skyline GT-R R32- Few cars were ever as dominating as the original Godzilla. There had been Skylines before. There had been Skylines that won races before. But never before had there been such a force of total and complete of overpowering automotive excellence from Nissan. A twin turbo inline-6, all-wheel-drive, 4-wheel-steering, and all other kinds of electronic gadgetry and technology came together to produce one of the greatest cars to ever be raced. It won the Japanese Touring Car Championship from 1989 to 1993, winning every race it entered in both the JTCC and the FIA N1 Super Taikyu. But the 1989 ownage of JTCC was not enough for Nissan, they travelled to enter the Australian Touring Car Championship. At that point the Ford Sierra RS Cosworth ruled the ATCC, but that all changed when Nissan brought their GT-R and took the series for 3 straight years. The GT-r dominated the Pacific touring car championships so badly that it didn't just lead to a change in the rules, it led to the demise of both the Japanese and Australian touring car cups.