The fastest car in GTA 5 Online differs from one vehicle class to another.
GTA 5 Online players should know that the Vigilante is the fastest car in the game. Predictably, that would also make it the fastest supercar in GTA 5 Online. However, it's also interesting to look into the fastest cars for the remaining vehicle classes. As this article's name implies, this list will only go over cars (and not all vehicles).
Popular GTA 5 Online YouTuber, Broughy1322 has compiled data for every vehicle in the game. One of the most valuable pieces of data is the car's top speed, as it's a more accurate measurement than what the game tells the player. Emergency, military, off-road, and utility cars won't be listed, as they're slower than other vehicles in that same vehicle class.
What is the fastest car in GTA 5 Online: Every vehicle class
Vigilante for Supercars [147.00 mph (236.57 km/h)]

This vehicle is the fastest car in GTA 5 Online (as of the Los Santos Tuners update). Its natural top speed is 147.00 mph (236.57 km/h), which puts it well ahead of every other car in the game.
The Vigilante costs $3,750,000, so it's also one of the more expensive cars in GTA 5 Online.
ZR380 (Arena Wars) for sports cars [140.50 mph (226.11 km/h)]

Arena Wars vehicles tend to be among the fastest of their respective vehicle classes. It's no different with the ZR380, as all three variations (Apocalypse, Future Shock, and Nightmare) boast an impressive top speed of 140.50 mph (226.11 km/h).
All three versions cost $2,138,640 but can be bought at a Trade Price of $1,608,000.
Toreador for sports classics [135.25 mph (217.66 km/h)]

The Toreador is one of the most iconic cars in the modern metagame. It's a powerful weaponized vehicle that can go up to 135.25 mph (217.66 km/h)
The Toreador costs $3,660,000 in GTA 5 Online.
Imperator (Arena Wars) for muscle cars [132.75 mph (213.64 km/h)]

The Imperator (Arena Wars) is the second Arena Wars variant of a car in this article. It's the fastest muscle, as it can go up to 132.75 mph (213.64 km/h).
The Imperator (Arena Wars) costs $2,284,940 but can be bought at a Trade Price of $1,718,000 in GTA 5 Online.
Toros for SUVs (127.50 mph [205.19 km/h)]

SUVs are deceptively fast in GTA 5 Online. The Toros is the fastest in this vehicle class, and it can go up to 127.50 mph (205.19 km/h).
The Toros only costs $498,000 in GTA 5 Online.
Issi (Arena Wars) for compact cars [124.25 mph (199.96 km/h)]

There aren't too many compact cars in GTA 5 Online, but the Issi (Arena Wars) boasts an impressive top speed of 124.25 mph (199.96 km/h). Like with other Arena Wars vehicles, it comes in three variations: Apocalypse, Future Shock, and Nightmare.
The Issi (Arena Wars) costs $1,089,000 in GTA 5 Online.
Schafter V12 for sedans [124.25 mph (199.96 km/h)]

Interestingly enough, players can use the Schafter V12 in races for both sports cars and sedans. It's not the fastest sports car around, but it is the fastest sedan, with a top speed of 124.25 mph (199.96 km/h).
The Schafter V12 costs $116,000 in GTA 5 Online.
DR1 for open wheel cars [124.00 mph (199.56 km/h)]

Open wheel cars have terrific acceleration and handling, but their top speed isn't as fast as one might assume. However, 124.00 mph (199.56 km/h) makes the DR1 the fastest open wheel car in GTA 5 Online.
The DR1 costs $2,997,000 in GTA 5 Online.
Windsor/Windsor Drop for coupes [118.00 mph (189.90 km/h)]

Both the Windsor and Windsor Drop possess the same top speed of 118.00 mph (189.90 km/h). However, the Windsor costs $845,000, whereas the Windsor Drop costs $900,000.
Cerberus (Arena Wars) for commercial cars [106.25 mph (170.99 km/h)]

Commercial cars aren't known for their top speed, so it should be no surprise that their fastest vehicle only goes up to 106.25 mph (170.99 km/h). The Cerberus (Arena Wars) is available in three variations: Apocalypse, Future Shock, and Nightmare.
The Cerberus (Arena Wars) costs $3,870,300, although it can also be bought at a Trade Price of $2,910,000.
Gang Burrito for vans [105.25 mph (169.38 km/h)]

Most vans are notoriously slow in GTA 5 Online, and the Gang Burrito doesn't deviate too far in this regard. It has a top speed of 105.25 mph (169.38 km/h), which is the fastest among all vans.
The Gang Burrito only costs $86,450 in GTA 5 Online.
Taxi for service cars [102.00 mph (164.15 km/h)]

Although the Taxi cannot be purchased in GTA 5 Online, it's still the fastest service car in the game. It boasts a top speed of 102.00 mph (164.15 km/h), which is faster than the top speeds of some vehicle classes.
Guardian for industrial cars [100.50 mph (161.74 km/h)]

There aren't many purchasable industrial cars in GTA 5 Online. The Guardian is the fastest of this vehicle class, as it can go up to 100.50 mph (161.74 km/h).
The Guardian costs $375,000 in GTA 5 Online.