My own personal opinion and recommendation is based on the feel and feedback when manually focusing...
I prefer to manual focus with older gear-driven lenses, rather than the newer fly-by-wire lenses. There's a feeling of disconnect for me with modern lenses that have in-body motors and freewheeling focus rings that have no hard infinity or close stops, and that don't offer more resistance when turning the focus ring. So for that reason, my recommendation for someone keen to manually focus all the time would be to consider a mirrorless camera that can mount any manual lens mount via spacer rings...preferably a mirrorless camera with a high-quality electronic viewfinder and focus peaking, which offer a good, reliable way to quickly manually focus on subjects.
I have manually focused on my DSLR only when using older body-driven focus lenses that have better physical feedback, but find that it's not quite as nice with the DSLR and optical finder for me because I don't have a split-prism focus screen that would be more helpful. So most of my manual focus work is done with my NEX-5N, via various manual mount adapters, and an attached EVF using focus peaking. I use Konica K/AR lenses, Pentax K mount lenses, and Leica M mount lenses all via simple spacer adapters. I like that I can use the camera in three different modes - M, A, and S priority, as well as even using the scene modes and special function modes like HDR and multi-frame stacking and panorama. The NEX meters through the lens, so you can shoot Manual mode, choosing the shutter speed and ISO in camera and aperture on the lens...you can shoot in Aperture Priority, setting aperture on the lens and letting the camera choose shutter and/or ISO...or you can shoot S priority, setting the aperture on the lens, shutter in the camera, and letting the camera choose ISO.
The focus peaking makes manual focus fast and easy...plus you still have the option to magnify the view on the EVF to help fine-tune the focus in critical situations.
Sony DSLR-A68 / Sony 18-250mm / Minolta 50mm F1.7 / Tamron 150-600mm / Minolta 300mm F4 APO
Sony A6300 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / FE70-200mm F4 G OSS / FE70-300mm F4.5-5.6 G OSS / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses