Looking over the past year at my killboard stats I see a smattering of various ships: 3 x in a Raptor, another 3 in a Myrmidon, the odd Ferox killmail, four in a Falcon, etc. But by far the most common ship that I appear on killmails in is an Onyx Heavy Interdictor.
There are several reasons for this:
1) Its almost always a handy addition to any fleet.
Whether you are a small gang or a large blob, the HIC brings a decent radius warp disruption bubble on a solid platform that is not as slow as a Battlecruiser or battleship.
2) Its useful in 0.0 and Low sec.
The script that turns the ~20km bubble into ~30km infini-point is pure win. Warp stabbed haulers beware the sensor boosted HIC on the gate.
3) Massive passive tank.
Certain versions (Broadsword, Onyx) can tank the current Doomsday device. 'Nuff said.
4) Cruiser agility and speed (well almost).
It can keep up with HAC gangs pretty well, but a lot more durable than a Light Interdictor.
For those reasons, I'm more likely to choose an Onyx when joining an ad hoc fleet than anything else, although I've have some fun in the Interceptors lately.
Here is my current setup:
The reason for the two Warp Disruption Field Generators is that it allows me to pop a bubble and target a ship to hit with the focused infini-point scram. That way targets that slip out of my bubble range are still warp scrammed for another ten km, giving the fleet the chance to take it out.