Been using mine for many years. Used to do my school work & even wrote my finals on it. Also used it though job school for writing reports & typing out dictation I has taken by hand. Always served me well. Mine is a wide carriage model from 74. 

In regards to serviceability & quality; Probably the Olympia SGE line. Those were made for heavy, professional use & with regular maintenance in mind. Olympia SGE 40 is probably the "flagship". 
Every other criteria is subjective. Personally, I prefer the feel of electric type bar machines over the Selectric. 

I mixed up the 3 & 4 in my post (sadly no editing here) but what you show here is exactly what I described. 
But honestly I think in practice a pinwheel calculator is better suited for anything more then adding & subtracting. Or just in general. These adding machines make nice display pieces tho, & are very satisfying to use. Ive been tempted to get another few more then once, but will keep it at the 1 for now. 

With a trick 2-species adding machines can also multiply. 
Here is a video on the matter: https://www.youtube.com/watch?v=qzUSOc3XtJI&t=775s

Its in German but perhaps it'll be informative despite. 

Thats where the phase "to be in the black" comes from. 
I might be able to service a chronograph but these things scare me. I dont really touch them, apart from superficial cleaning.