Unfortunately, unlike other GTA's this one doesn't have many if any fixed spawn points for cars. However the Banshee should be plentiful and easy to get.
Other cars like the Infernus, or the Super GT are a pain in the neck to find.
Anyway, go steal a Turismo from the Dealership that has nothing but Turismos in Algonquin. Then just drive around Algonquin and I'm sure a Banshee should will spawn around you. .
SPOILERS... Alternatively you can wait until you get the message from Stevie to get him one, and before delivering it to him, take it to one of your safe houses and park it in a parking space. Then return to the area Stevie said the car would be in, and another one will have spawned. END SPOILERS
As for best car depending on your driving style this may vary. I absolutely love the Dukes but its obviously not the fastest.
The Infernus is pretty much the fastest. With the Super GT in close second.
For Races I normally use the Turismo or the Comet.
"You know what time it is? It's clobberin' time..." "Look who just sprayed ya with a can o' whooppass"
well... when i do see them, i'm usually in algonquin on the east side on albany ave, south of Lancaster by the car dealership. sometimes when i go to Bohan via frankfort ave and on that street just south of the bridge is a spawning haven..... so if ur driving a banshee (for profit) that's a good place to go