35mm Sigma 1.4
Primes+FF = why did I ever shoot crop?
Yep, that's pretty much the gist of it.
If there's one reasonably-priced standard zoom I really like on the 6D, it's the 24-105L. They can be had refurbished at very good prices these days. This is also a lens that will hold its value very well. I love my primes but sometimes one "do-all" lens with a stabilizer is awfully convenient when walking around. This is pretty much my only zoom.
But (depending on what the OP is shooting, assuming he's still reading this) you don't really need the zoom if you get enough primes and you're rarely in situations where you don't have time to change lenses. If you're just starting out, I'd say this is a great prime collection that won't break the bank:
- Samyang 14mm
- Canon 40mm pancake
- Canon 85mm f/1.8 USM
And maybe add a Canon 28mm f/1.8 USM, though around the 20-24-28mm focal length you might be better off waiting a while & putting more money into a better lens later as there are a lot better options out there, but none of them are under $500.
Alternatively, if you don't want to go as wide as the Samyang you could put a little more money into a Voigtländer 20mm Color Skopar, which gives you a nice 20/40/85 kit (doubling your focal length each time you change up). Also two of them are pancake lenses, so the kit of all three is about the same size as a 24-105L <grin>. Obviously this is what I have (plus a 135L and 1.4X TC at the long end, and a Zeiss 35 & 50).
I shot a 10D for 5 years, then moved up to a 40D & used that for 5 years, and just got a 6D at the beginning of the year (so that should be good for at least another 5 :-). Ever since I started buying lenses I always made sure not to spend any real money on anything that wouldn't work on FF (the GF's crop camera has an 18-55 IS and 55-250 IS but I didn't spend more than $300 for the pair of 'em).
This may sound funny, but this is the reason that I stuck with Tokina for ultrawides. Both the old 12-24 f/4 and my current 11-16 f/2.8 are technically "crop" lenses, but unlike the Canon 10-22 the Tokinas have a normal EF mount. At the long end they both work just fine on FF cameras (I tested this back in the day on my old EOS 1N film body; the 12-24 didn't vignette appreciably until 20mm or wider). So my Tokina 11-16 f/2.8 is now a 16mm f/2.8 on the 6D, which is still quite a nice lens on FF (pretty much the same deal as the Samyang above, except the Tokina has AF :-).
Tokina does have a very nice FF ultrawide zoom, but that's a lot larger, heavier, & pricier of course.
BTW you really want to avoid the old micromotor lenses like the "nifty 50" f/1.8 if you have any intention whatsoever of shooting video or using live view on your 6D. At least with mine, the contrast-detect process and the old AF micromotor just seem to fight with eachother and never really achieve focus. I suspect this is why they developed STM focus drive as an alternative to the old micromotors.