As Josh stated, it is a very personal decision. For landscapes, most would agree there is a sweet spot from 20-35, and another from 70-135. On the wide end I've had several of the above-mentioned lenses, including the 17-35, 16-35, 24TS (v1), 35f/2, 35f/1.4, 24-105. I was recently debating between the 24TS II and the Zeiss lenses. I ended up with the Zeiss 21 and 35 for a few reasons:
1. For relatively light but still very sharp combo, the Zeiss 35 and Canon 90TS is tough to beat. Same 58mm filters, moderately heavy but not too big.
2. I think the 24TS II is a great lens, but it's big.
3. I still have the original 24TS. While not as good, I can use it as a specialty lens instead of the everyday workhorse.
4. Autofocus is not important to me.
My normal kit has become the Zeiss 21 & 35 plus the Canon 90TS and 70-200 f/4 IS. Someday I'd like to add the Zeiss 50 macro.