There is no stealth in space. Projectiles that are hotter than the background show up as hot spots, those that are cooler as cool spots, and there is next to no "cover" on scales much smaller than a globular cluster.
So you don't care if your weapons emit.
A problem with any kind of directed energy weapon is that inverse-square law. Their power goes down with the square of range.
A problem with almost any kind of kinetic or mass-based weapon is that the enemy can see it coming and dodge, and they are slow.
On top of that, you can random-walk, and at non-trivial distances make any kind of aiming a crap-shoot.
A plus side is that most mass based weapons (except, say, black hole bullets) experience time, so they can have sensors themselves and redirect where they are going.
So what you want is a fast weapon (reducing enemy counter-battery and dodge time) that can maneuver (hence mass-based).
So one idea is a microscopic antimatter Bussard ramjet missile. It converts interstellar hydrogen into propulsion using magnetic fields and its own mass. It will glow ridiculously brightly as any interaction with the interstellar medium causes particle annihilation.
As a staged missile, you start off with a conventional matter-antimatter converter launcher that gets the entire thing up to speed (perhaps powered by a laser up its backside). When it runs out of speed, it uses matter-antimatter converting technology to convert its payload missile into an antimatter one and lets it fly away from the "sabot" stage.
Such a weapon can reach a good fraction of c, maneuver as it approaches its target, and if it appears to miss can self-destruct in order to bombard the target with high-energy antimatter dust.
It will also be very bright. Maybe too bright, as it might be visible mainly in gamma rays. But the engine itself might glow in the visible spectrum.
Another idea is black hole munitions. Micro black holes are another total conversion engine you could attach to a missile; they'll put out the energy of a nuclear bomb every second (the shorter their lifetime, the brighter).
Throw some big science words at it; spin a micro black hole up to ring singularity speeds and charge it to permit coupling, and use it to accelerate propellant. Have three such black holes in a metastable orbit, and cause them to shatter as you approach your target, creating massive gravity waves and a directed neutrino pulse.
Going further, your weapons are Alcubierre drives. They form an event horizon round themselves artificially, which in turn generates hawking radiation based on the steepness of the curve (which is bright). The munition literally breaks the speed of light and distorts space as it goes through a target, causing destruction. Of course, this technology also permits communicating with the past via bullets, so "shooting enemy ships" is sort of like using a nuclear bomb as a paperweight.
You could have a gravity-munition that does space warping without the ability to break light speed, and has a similar near-singularity psuedo-Hawking radiation glow.
Another exotic idea is controlled false vacuum collapse. The universe's laws of physics have done something similar to "phase changes" when matter goes from gas to liquid to solid in the past (during the period near the big bang). Some have posited that our current vacuum state is not actually the lowest energy state, but might be metastable; what we call the vacuum is actually a false vacuum.
In that theory, the state of the vacuum can spontaneously collapse into a lower, more stable state (which would correspond to a change in the laws of physics, basically). Such a change could then propagate at the speed of light. Nobody would know it was coming, but it would literally rearrange the everything.
What if there was a lower energy somehow unstable vacuum state that you could only arrange if you carefully encased it in exotic space-time geometry? Like, imagine a glider in the game of Life; it would propagate in one direction, leaving behind normal space, but messing up whatever it hit? The exotic space-time geometry that encases it might decay at a rate of a Planck distance every 10^10 Planck-distances traveled.
In the game of Life, "spaceships" (self perpetuating constructs that move) travel at a fraction of the speed of light in Life (which is 1 square per iteration). Such a complex bit of space-time geometry might also travel at a fraction of the speed of light.
Then, as it tears apart the laws of physics then they snap back, energy could be released, causing a bright glow. We could have it break conservation of energy (perhaps it ends up interacting with Dark Energy, changing the rate of expansion of the universe, in order to fuel itself), or it could just fuel itself using the interstellar medium.
The interstellar medium is 1 proton/cubic centimeter. At the speed of light speeds, a 1 meter radius total conversion puts out 141 078 Watts.
Based off the Stefan–Boltzmann law, the color temperature is 1251 K -- the color of glowing red iron.