There are currently only a few classes of Necron ships that are known to exist, but are generally considered to be more than a match for most the other species on a class for class basis.
Tombships Cairn class Tombships are the largest (regularly) encountered vessels in the Harvest Fleets. They are extremely powerful, easily capable of defeating any of the other species battleships (the largest common ship type in the fiction).
Harvest ships Scythe class Harvest ships are the "workhorse" of the Necron fleets as they have been present in almost every encounter with Imperial forces, and like other Necron vessels are more than a match for similar class vessels. Their armaments are similar to the Tombships, although being less powerful and lacking the Sepulchre.
Shroud Shrouds are designated as light cruisers and their main function seems to be surveillance and recon more than fleet engagements.
Raiders There are two classes of Necron Raiders classified as Jackals and Dirges. Jackals are the larger, more heavily armed of the two with a Lightning Arc and a Portal, while the Dirge is the smaller and faster of the two, being armed only with a Lightning Arc. The speed of both ships (the Dirge in particular) is sufficient to outrun even torpedoes and bombers of the other species.
some info about necron ships