If I have to narrow it down...
1. Memory foam topper
2. "Cafe" dinette table (bought spare table and cut down to a two person table which opens the space up a lot - it's like adding a slide)
3. Sewer hose storage under coach (made from 6" PVC pipe)
4. Drinking water filter at kitchen sink
5. Vents: Maxx Air Fan in vent over bed; Camco vent covers on all others; Vortex fan upgrade in bathroom
5. LED lights throughout
6. Nightstand shelf in corner bed
7. Solar system (632W) with charging station (12V outlet strip installed in shallow cabinet during solar install; previously had no 12V outlets in coach)
8. Awning lock (peace of mind)
9. 
Standing desk (not that I want to work on vacation, but slick when needed)
10. Rear view camera (to see the toad)
Things I'm considering:
-air bags (our RV leans)
-upgraded (double pane) windows; particularly above bed
Did the Oxygenics, too, but installed before ever using the shower so I have no point of reference for how much better it is!