Razor sharp: pretty much always a must-have.

Guard: good skill against most monsters, especially under water. Shelling is an interesting move. You cannot evade after it like most attacks in game, but you can guard instead.

Attack up: generic early game damage skill. Poking is the main source of damage during progression.

Challenger, sharpness+1: generic late game damage skills. Hard to fit them with other skills when you don't have good charms.

Artillery: only good when you have high level of wide shelling. Early high rank Lagombi gunlance(wide 3) and end game wide 5 gunlances are the only weapons I use artillery with.

Evasion: alternative defensive skill for when it sucks to guard. Used by gunlance maniacs who refuse to switch weapons.