The map in FH5 is by far the best, more verticality than 3 or even 4, more biomes than either too. Horizon 4's map suffers from some pretty poor road layouts, shortcuts through the forests being non-existent and by it's weather system which is either rainy or snow conditions.
As a whole I still prefer FH3 the most though, 4 and 5 have become way too polluted with neon jizz and absolute shit characters who never shut up but for outright driving FH5 has the best flowing map with clear paths through woods, roads that line up to each other allowing you to jump roads easily and the most well laid-out map overall. The track editor is also just nuts.