Good mid-game weapons

Wave motion Gun is very good at long range and available at Lvl 40 Ripper Beam Tech area. I normally go for this.

Nul-Space projector is quite good, because it skips shield and armor, but i think this is more End-ame than mid-game.

Best regards,
Pit Fiend

I don't use Depleted Uranium Cannons much.
I go for Anti-Proton Beams (Energy Stream Weapons) as soon as possible.
They get better potential range, and don't need ordnance.

I also go for Torpedoes ASAP.
Anti-matter torpedoes are a good 'cheap' weapon, good if you want a seeking weapon without the high cost (and lesser range and damage) than the Capital Ship Missile (CSM), but it is a little smaller and faster firing.
Quantum Torpedoes are cheaper and have equal range to a CSM and they fire faster also, giving them better overall damage in the long run.
Gamma Pulse Torpedoes fall somewhere between the above two.

Weapon Mounts.
Not actually a weapon, but useful fo weapons.
Having a 50% bigger gun with 100% more firepower can be handy if you got good accuracy on your ships. With bigger mounts, you can get something like three times heavier and five times the damage.

Point Defense Weapons.
The Point Defense Blaster has a good rate of fire, making it good for not just taking out seekers, but also for shooting down fighters, drones and satellites.

Wave Motion Gun
As Pit Fiend said, it has long range. It's range is on par with CSMs, and it has high damage to boot, the only down side being the large size and slow rate of fire, but invaluable in a mount on a base, WP or sat due to the range/damage boost.
Massive Mount a Wave Gun on a Dreadnaught or Base Ship and they can more or less crush smaller ships in a single blast.
Massive Mount a Wave Gun on a Base or Large WP, and you got the ultimate defensive weapon that can possibly crush shields and shred armor of ships before they can even get into CSM range.

I usually go straight for Phased Polaron Beams as my standard weapon, then use Plasma Missles/CSMs for range, then get the null space projector, then near the end I use Temporal Shifters and Wave-Motion Guns.