Terrible skills: Concentration (very situational, small boost), Luck+4.
Top Tier: The various Expert skills except for Bow Expert. Essentially, Weapon Saver lets you use limited powerful weapons, but Expert skills are such a significant boost that it's comparable to using a powerful weapon. For example Silver Sword on an Expert has almost as much attack as Mercurius, with the additional benefit that you don't need to wait until A rank to use the Silver, you don't have to take the Mercurius away from another unit, and an Expert can still wield Mercurius occasionally for really tough enemies even if he can't use it all the time. So Expert skills should be close to Weapon Saver, except Bow Expert which is more limited.