Earl of Sandwich is great but its not convenient when you need a quick lunch while in the parks. Here are some good sandwiches I have had:
MK:
Paninni's at Village Haus. I was pleasantly surprised since this place gets crummy reviews quite often but my sandwich was really good.
Columbia Harbour House: Ham and broccoli slaw and the tuna sandwich are both wonderful.
The grilled chicken wrap at Pecos Bills is very good.
At AK:
The sandwiches at Pizzafari are good, way better than their pizza, especially the Italian chicken one.
Tusker House chicken sandwich was great.
The smoked turkey sandwich at Flame Tree was tasty, its kind of dry so I put some bbq sauce on it!
At Epcot:
I don't actually think I have ever had a sandwich at Epcot. My friends got some pretty decent looking ones at the food court in the Land. We had already eaten in Japan so didn't eat there.
At MGM:
Haven't had a sandwich here either except at Sci Fi which has a wonderful reuben. Due to the crummy cs options here we have almost always opted to eat at Sci Fi or Prime Time.