Resistance exercises that develop optimum human function:

The must do's:
A deep knee bend (squat variants: zercher, front, overhead, back, goblet... lets choose front)
Press
Clean

The deep knee bend of squat builds lower body strength, the press likewise for upper body.
The clean developes power in the entire body. Bill Starr wrote that athletes need only use the 'big three' exercises: back squat, power clean and bench press.

Condensing this:
Clean & Press
Front Squat
Get good at these and everything in life will be easier. These two exercises seem to have carry-over into everything.

Bryce at www.ironsports.tv advocates what's been branded the 50/20 method. Train 4 days a week (eg mon, tu, th,fr). Select a lift or two (clean and press, front squat). Put some weight on the bar and set a timer for 20mins. Do 50 reps or more. If you manage 50, put more weight on next time. If you get less, keep the same weight and try again next time. Alternate the exercises so you end up clean and pressing twice a week and front squatting twice a week.

Let us expand on this list:
Swing
Deadlift

The swing done properly provides all the work the hamstrings ever need, do 10 heavy swings then see how springy you feel.

The deadlift is in my opinion a safety exercise, it builds the back musculature like nothing else. Heavy deadlift loading strengthens the entire skeletal system. Being killed is much more difficult if you deadlift. A strong lower back is useful in rugby, many people ruck with rounded backs which is weak. This does not happen if you have a strong back.

Pullups and Farmers Walks
Deserve a mention as completeness exercises. Do them whenever. Farmers are best done with a friend.