Well, technically, the "best" car to start out with would be the Viper Concept for getting all gold in the Super License tests.
Like Solid said, the RX-8 is a good thing to get first, because you get it for all gold in the A License, and the A License is the easiest one to get all golds in. The RX-8 is a really good car. It's good enough to win the Polyphony Digital Cup (which requires an unmodified car), and doing that will get you the best car in the game, the F094/S (although you may have to save and reload until you get the F094/S).
Any of the free cars are good starter cars, but they're all pretty tough to get. If you don't want to bother getting all golds in any licenses, The Esperante GTR-1 is a free car you get for completing Time Attack in Arcade Mode. I don't like that car too much, though, and Time Attack is the hardest part of the game (unless you're going for license golds).
If you just bought the game, you'd be better off just getting some practice running races. If you're just looking for something to buy with that $18,000 you start with, there are only a few cars that you can get with that:
Daihatsu Mira TR XX Avanzato R: $11,140
Daihatsu Storia X4: $13,900
Mazda Demio GL-X: $14,660
Mazda Miata MX-5 (1989): $17,000
Mazda Miata MX-5 (1993): $16,900
Suzuki Alto Works Suzuki Sports Limited: $12,220
Toyota Vitz Euro Edition: $12,880
Toyota Sprinter Trueno GT Apex: $13,550
Toyota Vitz RS 1.5: $14,530
Chrysler PT Cruiser: $17,980
Volkswagon New Beetle 2.0: $15,930
Of those, the best car is the Toyota Sprinter Trueno GT Apex. It has more power than any of the other cars, is lighter than most of them, and leaves you $4,450. You should change the oil and spend $1000 on a Racing Chip, then save the rest of the money for some better tires. The only settings you can change are Active Stability Management (turn it to zero) and TCS Controller (turn it down to 2). You shouldn't have a problem winning Sunday Cup, and the Clubman Cup is also pretty easy if you don't hit too many walls, although the Toyota MR-S S Edition shows up there sometimes, and he'll get ahead of you if you don't slam him into the wall a couple times on the corners. If you're having trouble, you still have the money to get a Semi-racing Muffler.
The $16,900 Mazda Miata MX-5 is the second best car you can buy, and a lot of people like it. You could maybe argue that it's just as good as the Trueno, but if you want this car, you can get it free by winning the Clubman Cup, so you should save your money.
The Daihatsu Storia X4 is also a good car if you don't plan on keeping it very long. It has the best power to weight ratio of any car under $18,000, and it handles pretty well, but it just doesn't get much HP boost when you upgrade it.
) and get the Panoz, as Bollocks explained...then you're set to get a nice car, run the Super Speedway Enduro till you get an F1, then do the Beginner then Amateur Series (well, that's if you want to beat it 100% without any challenge...
