>>2045085 (OP)A&P here.
Neither a Boeing nor an Airbus are the best engineered as there have been issues with the A320, A330, A340 and A220 in the past and present.
If I had to guess, the best engineered is the L1011 (used to work on the ATA ones years and years ago), the design was very forward thinking and while there was a lot to be done in order to keep them flying, they were not necessarily hard to work on. I kind of liked working on DC-9s, MD-80s, MD-90s and MD-95s, but people with large hands will struggle to work on the tail jackscrew which operators the pitch control.
A few other notes are that commuter turboprops were either great or terrible, no in between. Saabs were a joy to work on, as were Dorniers, Jetstreams, Cessna 208s and Beechcraft 99s and 1900s. Twin Otters, Dash 7s and Dash 8 100s, 200s and 300s were good too, but Q400s were pigs, as were ATRs (reason why only FedEx flies ATRs for freight in the US).
I was never really tasked with Short 330s and 360s, but looking at them, they seemed alright, but they were always very tired looking airframes. Those which are still flying have been ridden hard.
If I had to pick a "best Boeing", excluding the 717, as that is a rebarded MD-95, then it would be the 757, followed by the 737-600,-700,-800 and -800, with the 727 being third. "Best Airbus" would be the early A340s and the A300/A310 family.
The worst aircraft I looked over was an An-12 from Ukraine. Told the company that it should not be flying and they refused to pay us for our work, for two or three years, so it sat on the airfield during that time. I would not be surprised if the aircraft in question was turned into a smoking crater between the time that I was working on it and now. All of the An-12s from Ukraine and Central Asia were like that, but we only received a handful of them from 2010 to 2015.
