2015 Ford Mustang VS Subaru BRZ VS Civic Si?

Which one should I buy for myself. I will use it mostly for driving pleasure. And which one will be the easiest to customize and maintain. And which one will impress my friends?

Civic would be by far the easiest one to customize and it would be the cheapest as well but if you want to impress people then get the Subaru, Mustang is just a expensive same ol' boring muscle turned sports car that every person making over 40k drives. Also, Civic's are very cheap on gas, so are most Subaru's, so those would be best for pleasure, Mustang's drink gas.

The Civic SI is a Japanese econobox pressed into duty as a sports car. It would be the most practical as a daily driver, but not as performance oriented as the other two. The Mustang is at the opposite end of the spectrum, especially with the optional 5.0 V8 engine. It's a beast of a muscle car forced into daily driver duty. If all out performance is your thing, this is the one to get. I'd avoid the 4-banger. You can get the same power for less money out of the 6, and since the 6 is not turbocharged, it will be more reliable in the long term. The purists will tell you only go for a V8, but frankly I think the price and performance of the new V8s take them out of the league of normal people with normal bank accounts and normal driving skills. The V6 makes as much horsepower as the 3v 4.6 GTs from a few years back, so it's still fast.

The Subaru BRZ splits the difference between the two. It's small, lightweight and fuel efficient, and would be cheaper than a Mustang GT. At the same time, unlike the Civic, it was designed from the ground up as a sports car.

If you want balls-to-the-walls performance, get a Mustang.
If you want a practical car that can still be fun & sporty, get a Civic.
If you want a good compromise between the two, get the Scion.