Even with a ressie at the Rose & Crown, you may not get an outside table with an Illuminations view. They're first come, first serve, and the people who have them tend to linger around for the show.
We've watched from in front of and to the side of Canada. Another place to try is the outdoor cantina in front of Mexico. But you'd have to get there early for a table close to the railing. We've also watched from the second floor of the big building in Japan. There's a balcony on the end by the water. It's part of the bar/restaurant there. If you like Japanese food, this might be a good place. (Or maybe just plant yourselves on the steps.) Otherwise, just find a good spot around the railing somewhere - and guard it.
The suggestion of just to the right of the castle for the MK fireworks is a good one. We sometimes watch the parade in Frontierland. Stand on the Saloon steps. Or watch it from the City Hall steps on Main Street. To be honest, we've never had a problem just finding a spot on the curb along Main Street.
DisFlan