I strongly recommend sticking with the stock size, usually 195/70/14 on most 123's with a 6.5 wheel. Going wider gives you a little straight-line advantage but sacrifices handling since the fatter tire will "swim" on the thinner wheel. The proper way to get a wider tires is to get a wider wheel too. A 205 tire should be on a 7.0 wheel, 215 or 225 on a 7.5, 235 on an 8.0, etc... just look at the factory sizes on later MB's. Fat tires on skinny wheels, IMNSHO, looks stupid.
With that said, I've heard a lot of good about X-Ones, long life, etc... but probably not all that great for performance use (I know, I know, 123 and "performance" shouldn't be in the same sentence). I've also heard good things about Kumho's tires, great bang for the buck there, I was planning on getting Kumho's next time for my 123.
