I have been repairing typewriters professionally for 45 years now.  The reason that I continued whilst almost everyone else gave up is that I genuinely like typewriters - it is a vocation for me, not just a means of earning a living.  The point of all this is that I get to try out and enjoy thousands of customers' typewriters without having to store them afterwards.  The best of all worlds !

If you hate your job, you are wasting approximately one-third of your life (one third sleeping, one third travelling to and from work, eating etc, one-third working)  If you enjoy your work, it doesn't seem like work at all (other than the occasional badly-worn machine that you spend hours and hours on and would like to throw at the wall ) and the time goes more quickly !

I have a number of favourites - for different reasons.  Possibly my favourite portable is a 1960's Olympia SF DeLuxe which belonged to my mother.  She bought it second hand when I was about nine.  It was the first typewriter that I ever saw close up and got to use.  I still have it and use it now and again for invoicing.  It has only ever been serviced or repaired by me.  Maybe my favourite standard is the Imperial 66.  My school opened traditional 'boys' subjects to girls and vice-versa, so I was able to learn typing - previously only for girls.  I learnt on an Imperial 66.  When I went for my job interview to become a typewriter engineer, the fact that I could type was a big point in my favour.  For my practical 'trade test', I was given an Imperial 66 and shown how to partially dismantle it.  I then had less than 20 minutes to reassemble it and make sure that it worked.  I did it in less than 20 minutes and got the job.  From a practical point of view, I like Olympia standards SG1 and SG3 a lot, but really admire the Hermes Ambassador.  Maybe the highest quality typewriter of all time is the pre-war Continental standard.  I do think the Olympia SF portables take some beating, but so does the early Adler Tippa and the Olivetti Lettera 22.  For heavy-duty portables I would have to pick almost any of the Olympia SM series, particularly the SMs 2-3-4.  The 1960's/'70s Adler Gabriele is a masterclass in how to build a modern typewriter with a minimal amount of parts without any loss of quality.

I agree that being able to type is a strength in typewriter repair. I've only just begun this trade in 2020 but I have seen and felt the benefit of typing out a page, deliberately, then at high speed. I feel like this is where the real diagnosis can occur because many touch typists (myself included) use a lighter touch, largely due to the takeover by computers, and if a type key arm can push and pull that force up to the slug without error, then the machine is in good working order. It's very different from hunt and peck testing of the keys which doesn't take into account the weakness or angle of the pinky fingers, to impart motion to type slugs at the ends, which tend to be the most finicky in alignment and movement. 
I think my SF DeLuxe is one of the most capable portables, and it's a great machine to work on. I think I prefer my Noiseless Model 7 for long sessions of typing. It forces me to really be consistent and has a wonderful "hum" to it when it's really going. One of these days I'll get to try a standard Noiseless, so long as I don't hurt my back moving it to and from the bench.

Phil Forrest

Without a shadow of doubt, my 1955 Triumph Matura.