Lasers at long range, railguns at medium and nuclear missiles at short. Lasers have a speed and precision advantage, not to mention they can't be shot down, but they can be reflected using right armor and have high energy requirements. Railgun slugs fly so fast that they'd be impossible to neutralize at medium range, and they'd deal greater damage that's harder to armor against. Nukes are good at ranges where a missile has little chance of missing (due to ECM) or being shot down, but they can do devastating damage, especially if we're talking fusion-based shaped charges.
Plasma weapons have a few disadvantages, most notably:
Plasma cools. This limits it's range to short-medium.
It's easily deflected by magnetic fields.
It requires a lot of power compared to other weapons.
Plasma could be used at extremely short range (essentially a glorified plasma cutter), but space battles have little chance of going into it. Alternatively, a plasma shield could be used for ramming, but that'd be absolutely a last ditch tactic.