According to the search results, there are several cars that are considered to be the fastest drag cars in Forza Horizon 5. Here are some of the cars that are mentioned across the sources:

• Rimac Concept Two
• Lamborghini Diablo GTR
• Koenigsegg Jesko
• Ferrari 59xx Evo
• COPO Camaro

Its worth noting that some of these sources have different opinions on which cars are the fastest, and some of the sources mention that almost any car can become a decent drag racer with the right upgrades and tunes. Therefore, the answer to the question "what is the fastest drag car in Forza Horizon 5" may vary depending on the players preferences and the specific upgrades and tunes used.