I think my choice for Gar's perfect moment is when he pops out of Raven's soul-self in The Technis Imperative, having snuck along for the ride with the original Titans. And says, "Um...Surprise!"
It encapsulates so much of how he relates to the other Titans: an unavoidable, irksome, yet lovable little brother who's loyal to a fault.
I also like the moment in the third Teen Titans run (collected along with the pages above in Beast Boys and Girls) when he reinjects himself with the sakutia virus, declaring that he likes his life.