It's better to look at and review a specific lens than a brand. Sometimes, the same line of lenses even differ (i.e. the original Canon 50mm Mark I was superior to the newer Canon 50mm Mark II). Sometimes it's extermely difficult to tell a version 1 from a version 2 of a lens, and the differnce can be from garbage to incredicle...
I own a Tamron 28-75mm F2.8 zoom lens (about $300). Optically, it's equal to the Canon 24-70mm F2.8 ($1500), but it's a full pound lighter and much smaller. However, the trade-offs include slower AF, lesser build quality, and it's not weather-proofed.