A lot of people like the Eminence Cannibus Rex for that amp. It's supposed to be a pretty clean speaker with nice, but smooth highs.
I have that exact same amp, and I use a vintage Jensen C12N. You can usually find them used for around $100 or so and I highly recommend them for that classic Fender sound. JBL D120F's are also popular for the same reason, but they can get kinda pricey. I'd avoid getting a new Jensen. They're the same in name only, and no where near as good of a speaker. If you can't find a decent original C12N, you might want to check out the Weber 12F150. I've heard they're a pretty good substitute.
I do think that replacing the speaker in those amps is a great idea though. They are amazing amps, but those original speakers are junk, in my opinion. Still, the question I'd ask yourself first is, what don't you like about your current speaker, and what do you want the amp to sound like after the swap? That answer and price will determine the best speaker for you.