The city I live in only has one major road (A1A: Collins Ave), but the Miami area is more interesting, so I'll do that. I wish I could post my own pictures, but I unfortunately killed my computer, by pouring coffee on the keyboard. These are ordered roughly in importance (for each section).
Miami Proper:
Biscayne Blvd (especially the Brickell Ave section).
-I think Biscayne is the heart of modern Miami (if there is such a thing). Starting in the Upper East Side, you pass the historic (if falling down) MiMo architecture district. Downtown passes by the Freedom Tower, the American Airlines Arena and the Bayside Marketplace. And Brickell Ave (south of the river) is the biggest concentration of high rises and skyscrapers.
Flagler St
-The historic center of Miami. It's the historic center. The Olympia Theater should be a more noted landmark. There are some other historic buildings in the surrounding blocks as well.
Calle Ocho (SW 8th St)
-The center of Little Havana. It's a bit of a sprawl for my tastes, but it's pretty important. Versailles Cafe (called the most famous Cuban restaurant in the world) is on Calle Ocho. The road becomes the Tamiami trail, continuing to Tampa through the everglades after it leaves Miami.
Honorable mentions:
NE 2nd Ave
-I'm not sure if I view this as a good thing, but this seems to be the center of Little Haiti. It's an area people tend to avoid, but if you're the adventurous type, it might be worth a visit.
NW 2nd Ave
-This is the best part of the Wynwood Arts District. It's still pretty rundown, but I like it.
I-95
-It's pretty boring to nominate the major east coast highway, but I like to watch all the lights wiz by as I travel through the city at night.
Miami Beaches:
A1A Collins Ave (also called Indian Creek Drive)
-It's the center of Sunny Isles, Bal Harbour, Surfside and most of Miami Beach. It has the hotels (Fountainebleau), bars and shops. I'm not sure if it's the best road on Miami Beach, but it's the center of the action. When it's Indian Creek Drive, you get a good view of the yachts and mansions across the river. That usually gets me psyched for a glitzy, superficial night out.
Ocean Drive
-The other major road on South Beach. It has the beach, the Art Deco Hotels and most of the overpriced, overrated bars and restaurants. However, it's the place to be if you want to spend a lot of money and see a lot of beautiful people. If you're not into that scene, it's still worth going during the day for the beach and a quick tour of the deco hotels.
Lincoln Rd
-The outdoor pedestrian mall of South Beach. Someone said this is where the locals go, but it's just as touristy as Ocean (just with a different type of tourist). It's more for shopping and people watching with restaurants that are a bit cheaper than Ocean.
Espanola Way
-A European and Hispanic-influenced road (alley?) on South Beach. It's almost all restaurants and bars here. It's pretty short and closed off to cars, so it's a good place for casual drinking. I prefer it to Ocean (I prefer almost everything to Ocean).
Washington Ave
-It's a lot like Collins--hotels, bars, clubs and restaurants. It's kind of grimy, but it has Mansion (one of the most famous clubs on South Beach) and also some cheaper places to grab a drink or a bite to eat.
Other good Miami-Dade Roads:
Coral Way
-One of the most underrated roads. It's lined with hammock-type trees, making the drive almost pleasant (it'd be nicer without all the honking cars). It goes right through the heart of Coral Gables. The Miracle Mile is a good collection of restaurants and is especially pretty around Christmas.
Old Cutler Rd
-One of the more relaxing roads in Miami.
Check out this description here
That might be it. Worst road? The Turnpike--ugh. 95 is pretty bad during the day.