I am going to suggest something a little left field
I have a kit that i pretty much take everywhere because its convenient and light and covers basically everything
I use the 5Dmk3 you have the 6D so both FF with great iso ability
I love the little voigtlander 20mm f3.5 color skopar SLII
they can be had for a reasonable price second hand
http://www.ebay.com.au/itm/VOIGTLANDER-COLOR-SKOPAR-20mm-F3-5-SLII-SL-II-WIDE-ANGLE-LENS-FOR-CANON-/190788561779?pt=Camera_Lenses&hash=item2c6be26b73its got electronic aperture control, manual focus but its got AF confirm with works well. Very well controlled distortion and is wonderfully made and all round a nice lens
it take 52mm filters (this makes buying CPL of 10 stop ND filters for landscape shots VERY cheap) a 10 stop B+W ND filter is only like $50 in 52mm!
move onto lens 2
the 40mm f2.8 shorty forty by canon
also a 52mm filter thread

so you can use those small cheap compact filters on this bad boy too!
its damn sharp and a brilliant general purpose lens of full frame, ligth and compact
this lens is about equal of the 24-70 f2.8 mk1 in the center and much sharper than that lens in the corners
The other lens i go everywhere with is my sigma 85mm f1.4
its just such an awesome portrait lens i cant leave it at home
but you could go with the 70-200 f4L IS instead if the extra reach is important
costs?
20mm voigtlander $600 new but can be had for $450 ish second hand
40mm f2.8 $150
sigma 85 f1.4 around $800
edit: i see you already have the canon 85 so may as well skip the siggy and go with the 70-200 f4L IS
about $1000