Stowaway/Traveler most, 175 EDF least. Size.
The sad reality of refractor ownership! Well any design really...
Not necessarily for any design...my most used scope by a wide margin is my 20" Dob and the majority of that observing involves travel to a dark site. Its capabilities make it worth the hassle, and make it hard to leavehome for DSO sessions.
Backyard duties for scopes are a different matter: shorter sessions, often just quick looks, and far fewer targets within reach of any given aperture because of suburban skies and seeing. Maximum portability gets the nod except when seeing is good enough to warrant more aperture.
Then there is choosing a "tag along" scope to bring along with the 20" at times for specific targets--e.g. very wide field, comets, some challenge objects for the aperture, etc. This mostly depends on what I might want to do, but portability is a key.
With respect to the original question:
In terms of refractors, my most used had been my ED80. It has served as a "seeing scout" in the backyard, as well as for moving around to see things very low in the sky or poorly positioned. The AT72EDII has somewhat displaced it because it is even more portable/versatile. The 72 gives some up for planetary though...and is on the shorter range of aperture to serve as a seeing scout.
Honorable mention in "most used" goes to the ST80 with RACI and 24 Pan that serves as a finder on the 20". Since I use it primarily in finder mode, it isn't employed as much for observing in its own right.
Least used is tough to say since I have a number of achro refractors with serious optical problems that are rarely used as a result. I also have a SW Evoguide 50ED with optical problems (this was the replacement, same problem as the first...probably need to give warranty a final go.)
Least used of my first line refractors is the 110ED, my largest refractor and the biggest impediment to it is size/weight/clunkiness in getting it outside. Now you might think that only reinforces the points others made above, and if limited to refractors this might be true... However, its problem is that it is too in-between in aperture compared to other types I have, and it takes just as long to set up. When the seeing is poor in the backyard there is no reason to hassle with it. When the seeing is good I typically want to employ the 8, 10, or 20" for more detail or to go deeper. I do tend to use the 110ED for some of this, but not when the seeing is particularly good and I have some time.
On the flip side, if I didn't have the larger scopes, then it would be the 110ED making the trip to dark sites as the primary scope, despite its greater bulk and set up time than the smaller refractors in altaz. And it would the the scope used in the backyard when the seeing was good.
Edited by Redbetter, 18 January 2021 - 02:27 AM.