Lenses
You cannot take a good picture with a bad lens. You can take some really amazing images with a poor quality lens, but if you want good contrast, sharp focus and attractive bokeh then you need a well designed and well made lens. Modern computer technology means that designing a good lens is within the grasp of any lens company, but getting that lens onto the market at an affordable price is another job entirely.
I am constantly disappointed by the quality of the lenses offered with what are otherwise excellent cameras, and the ubiquity of the zoom lens, and the need for high dioptric powers dictated by small sensor digital cameras just makes things even worse.
I like a good quality “prime” i.e. fixed focal length lens, with a decently wide aperture, so will often be using a 135mm f2.8 Zeiss, or the excellent 50mm f1.4 Zeiss “standard” lens.
I do use zoom lenses, and have a passion for extreme wide angle, started by my sadly departed Zeiss 18mm f4, which has been replaced by the surprisingly excellent Sigma 12-24mm zoom lens. I also use a Canon 24-105mm f4 L zoom lens for “general” photography. I occasionally use a 500mm mirror lens, but it leaves a lot to be desired in terms of quality, but is oddly useful on my infra red body.