Easy choice for hiking on a FF rig = the Canon 24-70 F/4 IS.
Small length for an FF zoom
Useful FL for hiking -- wide enough for landscapes but has some reach to crop / compress down-the-barrel trail views
Light -- I believe the second lightest L zoom behind the 17-40
Considerably sharper than the 24-70 2.8 Mk I and 24-105
IS is nice (but usually not much help in daytime hiking)
The 0.7x Macro mode -- despite the very short working distance -- is a nice functionality for wildflowers on a hike
That'd be my vote. I've taken it on a few hikes to good effect.