Let's face it, you could probably go for any gun in D1 as most guns in D2 are pretty lacklustre, but if you were asked to bring an exciting gun back, with all of it's perks, what would you choose?
For me, it would be Vex Mythoclast. If we could bring back the chase for the gun too, I would as it was the gun everyone wanted at one stage. It looks magnificent and it wrecked in PvP for a serious amount of time. It also sounded amazing (pew, pew, pew).
If it had to be a Legendary gun, it would've been Corrective Measure Machine Gun. I had an extended mag on mine which made it 100 rounds and it wrecked boss fights.