I would pick the deadlift mostly because I really cared about getting better at it. There were a couple of years where I almost exclusively trained the deadlift. And the total number of reps of the workout, warm-ups included was sometimes under 10 teps! Lol. I didn't lose any muscle at all, maybe a few millimeters here and there, too little to notice. I remember looking for a job and the interviewer asked how much I trained and I said, "just a little," because 10 reps a week of one exercise would be "very little" by most peoples' estimation. The interviewer, "the fuck you 'train a little', I can clearly see you lift alot." Deadlifts do activate most of the body at least a little. No one does deadlifts as a quad exercise but they are activated for sure. I don't know about pecs and arms, although more than a few times even my pec contracted so hard I thought I tore it.
It's also why I think you need very very little to keep whatever muscle mass you have, say 2 workouts a week. Deads, squats, curl grip pullups, dips or presses. Maybe a set for both the bicep and tricep, abs, calves although they are actually stimulates by deads and just walking out of the rack when squatting. With 3 fairly brief workouts a week you can for sure even grow, perhaps even optimally.
( as long as the hormones are there lol)]