I'm sort of assuming you're a fighter - because the other classes (usually) have only two-star proficiency, max. If you're thinking of grand-mastering (5-star proficiency) in one type of weapon, you'll probably be OK with ANYTHING

- hell I tried a fighter grandmastering in quarterstaff - a relatively sissy weapon for a buff fighter, and he was OWNING in every fight even with a +2 or +3 staff, and +4 Staff of Rynn was almost overkill...
You can get these godlike (Fighter) weapons fairly early on: Celestial Fury katana; Flail of Ages; Tansheron's short bow (not quite so early in the game); Azuredge axe (good alignment only), which MOWS through undead, especially in throwing mode - it's so powerful in this respect it's almost unfair... See for yourself what they do, you'll be amazed - get high proficiency in any of these, with a little mage and cleric backup, you'll be fine
Lilarcor 2H sword is NOT BAD for a fighter if you can put up with its yapping at all hours of the day... For other 2H swords, other Halberds, Bastard swords, Warhammers, you'll have to wait a WHILE to find the best versions of these classes of weapons.
And Impaler does ROCK.