Missionären
The missionary position already does a lot of the work for you, so the toys you bring into it don't need to be complicated. What you actually want is something slim enough to fit between two bodies without anyone having to stop and readjust. The vibrators in this category are picked specifically for that: low-profile, quiet enough to not kill the mood, and strong enough to matter. A lot of couples don't realize how much a small clitoral vibrator changes the experience here until they try one. Keep it simple, keep it close.