My personal favorite SLR line--for which I have loads of lenses--is Nikon.
The Nikkor lenses have generally (with minor exceptions) been optically amongst the best in class in whatever era they were released--and many of those, including a lot of the manual focus lenses, can hold their own even today in terms of character, color rendition, and more than acceptable sharpness (even though some may, in the latter category, not entirely please those who are more interested in pixel-peeping and bragging about measured resolution, instead of just making aesthetically good photos). In addition, Nikkor lenses--even the "cheap" E Series lenses--were for the most part built to very high standards, often much higher than their competitors. (When I lived in London, I mentored a number of young photographers. I had to explain to several of them why the front of their AF Canon 50mm/F1.8 fell off.) The "professional" Nikkor manual focus lenses are extremely well-built and most are also very high performers optically. While not all of Nikon's autofocus lenses were built to the same standard, many of their earlier pro lenses--particularly those with the crinkle paint finish--were mechanically very solidly built indeed.
But the big thing was the extraordinarily long maintenance of the F mount, and the ability to use MF lenses on AF bodies, and AF lenses on MF bodies, at least up through the introduction of G lenses. (Which still could be used in a pinch, although with severe limitations, on MF bodies.) That allowed me to build up a large collection of Nikkor lenses that could be used, almost seamlessly, on any of my film or digital cameras. And in fact I occasionally carried a small film SLR (normally an FE) and a DSLR, knowing I could easily use the same lenses on both. This saved me huge expense in not having to maintain two (or three) separate lens lines for different kinds of bodies from the same manufacturer, and allowed me to buy a greater variety of lenses for different purposes. And use them whether I chose to shoot digital or film on a particular day--if I chose, I could stock a bag with a set of lenses, and then merely switch out bodies to manual focus, autofocus film, or digital, depending on which I wished to use.
I'll have to re-evaluate, possibly, with the introduction of mirrorless. But formerly, whenever a new/young photographer expressed a desire to try out film, I always recommended a Nikon FE--it would provide easy-to-use full manual operation (with some automation in terms of aperture priority), for a "consumer" camera which could still be purchased second-hand quite cheaply it was very well built and highly capable, and they could buy lenses (I usually recommended autofocus lenses) that would work perfectly well on the FE (or autofocus film camera), but would also work totally compatibly on a Nikon DSLR. I know that other brands have their aficionados--for good reason, each has its particular strengths--but overall, given my desire to shoot a variety of cameras and both film & digital, Nikon worked by far the best for me--and gave me all the image quality I could ask for.