Sally Hansen's Insta-Dri topcoat used to be my go-to, but I don't like the "flatness" of the finish; it dries shiny but it doesn't have that thick, glassy, gel-nail look that I like. (If that makes sense)
Essie's Good To Go! is pretty good but I could only use half of the bottle before it got too thick and unworkable.
Lately I've been using Poshe and Seche Vite's topcoats and they are the best I've ever used! Actually I prefer Poshe at times, it seems to last longer for me but it is not widely available in my neck of the woods.
And I agree, a good base coat is crucial too!