There is no "best" weapon for shaman. It used to be any fast weapon, but no longer. In part it depends on how you spec and how often you group. If you get up to Stormstrike in the Enhancement talent line you'll get more use out of slow, hard-hitting weapons. You can get away with using a two-handed weapon more often in groups than solo; many shaman (myself included) really prefer having a shield when soloing. However, I also keep a staff around for grouping. My reasoning is a) staves have caster stats, and groups want me for my spells (especially healing), and b) I shouldn't need the shield because I shouldn't be getting aggro.
As to what type of two-hander or what type of one-hander, the answer is: whichever has the DPS and stats you like the most.