I have three lenses for my Canon 6D I consider essential. Life wouldn't be the same without these.
#1. My main carry-around use-everywhere lens is the Tamron 24-70mm f/2.8 with image stabilization. The Canon 24-70 f/2.8L doesn't have IS, and that pushed me over to the Tamron. Most people don't need f/2.8 on a lens this wide. f/4 should be plenty (and MUCH lighter) for most people. I take too many shots in the dark (ballroom shots, for example) and find I really need f/2.8. I find I really like having a quality wide angle option. I would buy this lens again.
#2. Sharing almost equal time with the Tamron is my Canon EF 100mm f/2.8 L IS Macro. This is one of the finest prime lenses made. Period. Abso-freaking amazing. I am sorry I waited so long to buy one. Wow. Just pile on a bunch of superlatives, because this lens deserves it.
#3. Getting less playtime since I got the macro is my Canon EF 70-200mm f/2.8 L IS II. Another world class lens. I bought this to replace a 70-200mm f/4 L IS that just wasn't fast enough for me. I can toss on a 1.4x extender and still have f/5.6. An amazing lens, but I have been using the macro more. This f/2.8 lens weighs a ton. You really feel it carrying this monster on a hike. The f/4 is far lighter and smaller, and I tried to get by. Really. But in the end I replaced it with the f/2.8 and I am glad I did.
On my latest hike through the Fort Worth Nature Center and Preserve I left the 70-200 at home. Shocking, I know, but I was fine with lenses 1 and 2 and (combined with a borrowed sling camera strap) was much more comfortable without the extra weight. Seriously, that 70-200 is a barbell. Thing must be lined with lead or depleted uranium. Dang it is heavy.