9. IKS Rotarran (Klingon Bird Of Prey)
Few ships in the entire Star Trek universe strike fear into the hearts of their enemies like a Klingon Bird of Prey. When one decloaks in front of your ship, there's a high probability that you're in for a fight that you are about to lose badly.
While a Bird of Prey, like the IKS Rotarran commanded by General Martok, is by no means the biggest ship in the Klingon fleet, they are one of the most effective and the most versatile. It filled a variety of roles, but was best known for its use in combat missions. There's a reason why it had the reputation it did, and why it was in the service of the Empire for almost a century.
There were two factors that made a Bird of Prey so deadly. First, it was heavily armed, featuring at least two disruptors and two photon torpedo launchers. But what gave Klingon ships the edge in any fight were their cloaking devices. You literally never saw them coming.
The only downside was that weapons and shields couldn't be used while cloaked. When a Klingon Bird of Prey decloaked, there was a brief moment of down time before they could put their shields up when the ship was vulnerable. It took a hell of a marksman on the other ship to take advantage of the window but it did happen.