I like the NYTimes op/ed section. But you have to pay to see it online now. (it's only $50/year, pennies a day...)
IHT.com is the International Herald Tribune, it's owned by the NYTimes and it's filled with good op/ed columns from foreign papers: Ha'aretz, Asahi Shimbun, El País, Frankfurter Allgemeine Zeitung GmbH, and JoongAnn Daily. The Wall Street Journal is good if you don't want to read my beloved liberal rags, but you'll have to pay for it, too. The Washington Post is good. Make sure you don't just read Op/Ed columns. The MCAT passages are varied in style and content so make sure your prepatory reading materials mirror this. There are natural science passages, and I think Scientific American is good reading for that. As you progress in your classes, start looking at more rigorous scientific journals, but for now keep it simple! There are liberal arts passages and any kind of boring reading about liberal arts will help you there.
Frankly, I think any reading will help. Let's just put it that way.