I don't think I could pick only one NES shoot-em-up.
Too many great titles.
There are a lot of overlooked shmups on the NES and one that I have grown to love lately is the NES port of Gyruss. I never played this until recently and I've been a bit addicted.
Do you play any shmups on the NES? If so, what do you like? I'm hoping I can find a few recommendations in here. So far, I have:
Gyruss
Gradius
Life force
Xevious
Xexyz (if this counts since its half platformer)
Section Z
I loved the NES port of Gyruss. I rented that and Ninja Gaiden at the same time (back in the day). It was so fun playing both of them (alternating between the genres).
I was surprised how often I have gone back to Gyruss over the years. It's a fun NES title.
Section Z, I remember mapping it out when I was in middle school, back in the day. I can't recall if I ever beat it, but I do remember getting frustrated with the mapping. I just wanted to beat the damn game!