and last but not least... most serious landscape photo competitions will disqualify you for doing such editings. for a good reason.
What reason? It's the exact same thing as using a grad ND.
Just google "Lindisfarne Boats". A great photo, but outside the rules and ultimately disqualified.
Every competition is free to have its own rules, but many landscape competitions apply FIAP "nature" type rules, in which "No elements may be moved, cloned, added, deleted, rearranged or combined. No manipulation or modification is permitted except resizing, cropping, selective lightening or darkening and restoration of the original color of the scene." Under these rules, using a neutral grad (or applying a similar effect in PhotoShop) is fine. Replacing whole skies isn't.
But there are many other competition categories where anything goes and extreme photo manipulation is often expected.
Back to the OP, my everyday kit consists of: -
Hoya NDx400 (a 9 stop filter and a viable alternative to the Lee filter for those who prefer screw in lenses)
3 stop ND
R72 Infra Red
When shooting B&W film, I also include Red and Orange. When shooting colour, I'll typically throw an 81B on.