best lens would probably be the 14-300 f2.0
+1 Oh, but wait, that was only mkI.
vII of the 14-300mm is also f/2.0, but adds a 4.0x built in tele-converter, and has 10 stops IS and USDSM (Ultra-super-duper-supersonic-motor) AF.
Naturally the mkII retains its status as a Canon EF lens; is a L, weighing in at 300gr, and uses a 58mm filter (because I asked Canon nicely with a 'pretty please' as I already have 2 Hoya filters of that size: CPL and ND).
Understandably, we all want to know when the mkII will be available. Well, mkII is said to be released along with the Canon 200-400mm f/4 1.4x IS USM, at 20% of the latter's cost. This is again possible, because I asked with a 'please' and said 'thank you for the realisation of a lens of this nature' in my email correspondence with Canon's CEO.
Ok... enough fun from my end! Seriously now... probably the 'best' lens (as a general purpose) is the 24-70mm f/2.8 USM vII - if you can afford that and need / want f/2.8. I use primes for true 'fast glass' so I don't use 'fast glass' zooms. The 24-105mm f/4 IS USM is a good & affordable alternative for the 6D. Or the 24-70mm f/4 IS USM if you want some decent close-up capability. The Tamron 24-70mm VR has also a large fan base.
I have the 15-85mm IS USM on my 7D, and love that lens. It's my go to lens for most all-round situations & travel. If I had a FF, I would probably go for the Canon 24-70mm f/2.8 vII on a 5DmkIII, or 24-105mm on a 6D. There are some great package deals going with 6D and 24-105mm at the moment. I do find 70mm at the tele-end limiting in a FF zoom.
Let us know what you decide, and enjoy taking some photos and sharing!