Although not a place one thinks of for Cuban food, the best Cuban sandwiches and mojitos that I've found in Seattle (or even out of a few places I tried in Miami) are at La Casa del Mojito on Lake City Way. http://www.lacasadelmojito.com/

They marinate the limes in their mojitos for over a day. Their service is quite friendly, but can be a bit slow. Still, it's a fun atmosphere with good food and drinks.