We can't drive GTA 6 yet. But we can read its visible road network and select the routes that seem to stand out from the rest. Speculation, obviously, and clearly framed as such: the routes below are selections, not road tests.
For the road network as a whole, see the state highways piece.
The Keys bridge route
The main route. Heading south out of Vice City across the bridges, island-hopping your way to the Key West stand-in. Water surrounds you on both sides for long stretches. Iconic enough that this will probably be one of the most frequently travelled routes in the game. It gets a dedicated page in the Keys bridge route piece.
The coastal interstate
From Vice City north up the coastline, past the suburbs and into the rural towns. A long drive at high speed with the ocean to the east. Probably the cross-map equivalent of GTA V's drive from Los Santos up the Pacific Coast Highway.
The wetlands cut
An east-west or diagonal route across the wetlands. A road through saw grass and cypress with water on either side. Occasional stilt houses. Less hospitable than the coastal route but more visually interesting.
The orange-grove run
A back way inland through agricultural Leonida. Narrower, slower roads. Trailer aerials show this type of road running between the orange groves and the small towns. This is the drive you take for the atmosphere rather than the destination.
What we can't pick yet
Mountain paths (Florida is extremely flat - see the elevation piece for more on that). Specific bypass routes around Vice City. Off-road or dirt trail routes - Rockstar usually ships these, but they aren't shown in the trailers. Traffic patterns - we don't know how traffic flows at rush hour versus off-peak.