When someone asks this question, the most likely obvious answer is "Oh it's Real Racing 3, duh." But is the answer really that simple? I mean, take a look at the competition.
First up we have GT Racing 2, by Gameloft. Like Real Racing, it's quite old now being released in 2013. Unlike Real Racing however, Gameloft haven't updated it for years (aside from some minor bug fixes.) Almost as if they've given up on it. The car list is OK, it has everything from a Fiat 500 to a Cadillac GT3 race car. It also has a good selection of Classic cars, and a few Concept cars. As far as mobile games go, the Graphics and sounds are pretty average, but the driving experience is decent. Not bad, but far from the best.
Also from Gameloft, we have Asphalt 8 and 9. If you are looking for a realistic racing experience, avoid both of these. But if you want a game where you can drive a car before it's even released in the real world, these are the games for you. Ill just list a few cars from each, and you tell me how many you've heard of. (And yes, they are all real cars). BXR Bailey Blade GT1, ED Design Torq, Lucra L148, Mercedes Biome and McLaren X2. The list goes on. The crazy car list, including Bikes AND F1 cars, combined with the insane stunt driving physics and ridiculous sense of speed, makes these games a LOT of fun. Definitely in the top 3.
If you look at some of the more recent additions, you will find two, very respectable games. You have Assoluto Racing, and Gear Club. Both of which are more realistic than both the previous mentioned games. Assoluto, gives a lot of Gran Turismo vibes, and definitely has the most realistic physics of any mobile game I've played. It is, by far the closest you'll get to a console game on a phone. The only thing that really lets it down, is the lack of cars. There isn't that many, and it's very Japanese heavy. Although they do usually add more with every update. So if you wait a bit, you'll have more to play with.
Gear Club 2, is actually a console game on the Nintendo Switch, which shows how good it is, but the first one is also on mobile. It is made by the people who brought you Test Drive Unlimited, so not only is it graphically very pretty, It's also very good. Even the sounds, especially for a mobile game, are pretty darn good. And then there's the body customization. There is so much to choose from. Dare I say it, It's almost as good as The Crew 2. It most definitely has console vibes to it. But, like Assoluto, the only problem, is the lack of cars. There's a lot of good ones, but not many unfortunately.
So as much as it pains me to admit it (partly because it's owned by gold digging EA) Real Racing 3, is the best one. There is over 200 cars, which is more than some Forza Games, and the car sounds, are mostly pretty accurate. It is very, very good. That can't be denied. Firemonkeys, have done a fantastic job, with RR3, and they should be very proud. Proud that they have created, the ultimate Mobile Racing Experience.