I have been using 1DX series bodies since the line was introduced and if you don't have it, the EF 70-200 f2.8 should be your first addition. Excellent performance (image quality and focus speed) across its "zoom" range. I shoot primarily sports and it is always on one of my 1DX III bodies at an event but it is a tremendous all around lens and one of Canon's best.
The EF 24-70 f2.8 complements the 70-200 f2.8 towards the wider end of the range and gives you great coverage of the most commonly used focal range when paired with the 70-200 f2.8. I was at my family reunion yesterday and used a pair of 1DX III bodies with the 24-70 on one and the 70-200 on the other which covered everything I needed from group to individuals both inside and outside.
A 50 mm is the classic prime in the normal focal length range and I switched to a Sigma 50mm f1.4 from my Canon 50mm f1.4 because I think it is a better lens in this focal range. I am very happy with this "Art series" lens from Sigma.
The EF-100 f2.8 is the choice in macro lens for this line, I have the IS version and I recommend it. The EF-100 provides excellent image quality but the focus speed is a bit slow since it is intended for detailed macro use but otherwise it is a good lens in the portrait range.
The EF-85 f1.2 is the EF equivalent of the RF-85 f1.2 and I have one. It does provide amazing images but it requires more finesse than most lenses to use at its wide open setting. Do it right and you get beautifully detailed images with a beautifully blurred background but miss the setup just slightly and you have garbage due to the very shallow depth of field with this lens wide open. Stepped down in aperture, it becomes easy to use but then you give up the reason why you bought this expensive f1.2 glass in the first place.
The EF 17-40 f4 doesn't get as much attention as some of the others but it is a beautiful landscape and group photo lens. I use mine frequently.
I have several of the Canon long telephoto lenses since I shoot sports (EF 200 f2, EF 300 f2.8, EF 400 f2.8, EF 800 f5.6) and those are all absolutely lovely lenses with fast focus and wonderful sharpness. My EF 400 f2.8 is my most frequently used of this group when shooting field sports and image quality is excellent, I think the EF 300 f2.8 SLIGHTLY edges it on sharpness but these are elite lenses that provide amazing images so the differences are minor.
Another lens I use frequently for daytime sports is the EF 200-400 f4 with integrated instantly switchable 1.4X extender. It is extremely versatile but doesn't have quite the focus speed or ultimate image quality of the long L series primes but it is extremely good.
No matter what glass you buy, take the time to calibrate the lens to the AF system in your body. It only takes a few minutes and Canon lists the process in their knowledge base, this is for the 1DX III but the procedure is the same for all of the 1DX series bodies: https://support.usa.canon.com/kb/s/article/ART176331
I don't use the exact Canon adjustment procedure and modify it slightly for speed and ease of use. When I buy a new lens, I will shoot test images using their procedure except I make multiple tests before checking the results on the computer by shooting a series of images with no correction and then both plus and minus 1, 3, and 5. I have never had L glass that needed more than 5 points of AF bias. From that initial set of images you can tell if the AF adjustment is perfect, if you need to use on of the chosen offsets, or if you need to run a second test interpolating now between the originals so use offsets of plus and minus 2, 4, and 6 and compare again. Using this modified process, I can perfectly calibrate a new lens to a body in a couple of minutes and I do so every time I add a new lens.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
