If by "Touring Car" you mean the normal WTCC cars, then there are many that you can be fast in. The BMWs are undoubtedly the fastest with a skilled driver, but the SEAT and Honda are the fastest out of the box. The Peugeot 407 is very good as well.
I think, the answer is depends on how to drive..
If drive it as an drag car..just straight forward, maybe Honda (FD)
is the fastest car.
But in racing, all of track has difficult corner...
In that point of view, I think BMW(RD) is the best.
but BMW is one of the difficult car to maneuver.
It really depends on the driver and the setup. If we used the stock setup and the same driver the C30, Accord and Leon would probably be quickest - they seem to be the best on Simbin setups compared to the other cars. With a few tweaks to the other cars though they can easily run the same laptimes as one another. Take the BMS or the 407, horrible on the standard setup but if you work at the setup quite a bit then you can easily get another second if not more out of the car.
To finish off my point, I think the Honda has good top end speed, the BMW has great acceleration of the line and the Volvos hold the corners really well. The seat is a great all-rounder and the Peugeot is surprisingly good.
The C30 has always been strong from what I've seen personally. The BMW can really struggle in race situations with bumps etc but has superior tyre wear, the Honda's brakes are excellent, although it's a little gutless, the Peugeot has an excellent ability to change direction, and the Leon is a mix of all. Although of course the new TDi is quick in a straight line and a pig around corners