I've put loads of thought into this and while it depends on the heist approach, i've whittled it down to just a few options.
-Bikes. Cheap, fast and manoeuvrable, provided you can get a good head start on the cops, you might just be able to stay out of their way for long enough to lose the heat just dont get caught in a fire-fight.
-It worked for the italian job so why not for GTA? getting an Issi would give you a size advantage so you could lose cops in tight spaces through parking lots or underground tunnels.
-Granger- 2 are all thats needed to haul a full server across the map and they're massive, too.
-Another idea i had was to load a supercar onto a flatbed. Use the flatbed cab to break through the bulk of cops then speed off the back and into the sunset.