Let us presume that the cost of the tickets of the routings for me to go to YOW directly to a European hub (+/- Canadian hub such as YYZ or YUL) and then to Madrid or from YOW to a US hub or non-hub city (eg BOS, ORD, EWR, JFK, IAD) and then to Madrid are identical.

Of note, I am a SuperElite with Aeroplan, and therefore a Star Alliance Gold member. I am a nexus/Global entry member. I also had a Priority Pass card (free entry for myself and one companion).

Obviously, from the previous paragraph, lounge access (both departure and also arrival, if possible) is important.

With nexus and GE; I do know that I can depart (to international destinations) from US cities on either US domestic airlines or on AC, and I am eligible to get TSA precheck.

But an important feature is qualify of service, and the quality of seats/beds.

I usually fly on AC's business class pod seats, that can configure to a truly private horizontal flat bed. I suspect that none of the competing Star Alliance carriers can offer all of that with a business class seating, but perhaps F class cabins can offer that (I am not interested in flying first class).

Putting it all together (and other considerations I can not think of), what do you think about AC, LH, LX - my main considerations. I have not really consider TK or SK - too far out of the way.
I might be amenable to UA, OS, TAP, or SN - but I remain to be convinced. I am not interested in LOT.