To echo Bill, the Sigma 18

To echo Bill, the Sigma 18-35mm is fantastic. Sometimes I want a little more range in focal length, so I use the Sigma 18-55mm f/3.5 on my 70D. It's slower, but as long as I know there won't be any super low light situations, I find the tradeoff to be worth it.

Another great option if you're just starting out with the 70D is Canon's 50mm f/1.8. For the price and speed, it's hard to say no to.

The last thing to think about is how loud the AF can be on the 70D. I'm considering getting an STM lens to eliminate the noise. My cam op used the 70D's AF for some BTS footage and the Sigma was so loud it was almost unusable.