Any sort of plasma weapon would diffuse into complete and utter harmlessness before it gets a hundred meters out, which is hilariously shorter range compared to basically any other practical weapon. This is because plasma is, basically, hot gas, which will expand into the vacuum of space and fizzle out almost immediately.
Even nuclear devices aren't as effective as you'd think. Without air to create a shockwave, what you get is basically a gamma ray/x-ray flashbulb, the intensity of which will fall off as the inverse square of the distance from the explosion as is standard for all point sources. Get it close enough and you'll get fun vaporization effects from the radiation impinging on the target's hull and possibly radiation effects on the crew, but you have to get it decently close to the target for that to work. Think under a kilometer, which is pretty close range when a tenth of a light-second (probably a reasonable range to hit someone with a laser without worrying about lightspeed lag too much) is 30,000 kilometers. Neutron bombs are better if the ship is crewed, since it doesn't take a lot of neutron radiation to kill the crew. A big (one-megaton) bomb might to able to kill out to, say, 300 km. You could probably get a missile that close to the target.
Lasers are nice and fast, but their effectiveness also falls off with range due to focusing difficulties unless you have a REALLY powerful laser, or a very good lens.
For a very, very good introduction to some ideas about rockets and space, go to
Project Rho.