Most people agree on Stealth Attack & bombard (1 rate of fire). As far as a/d/m - that's up to you, based on what timeline you place it in. I would think something with a fair attack, good defense, and minimal movement. Maybe 8/12/1 with 6 bombard (1 range, 1 rate of fire), Stealth Attack, Ranged Animation, Lethal Land Bombardment? I personally plan to use it in a way that minimizes the number of these units. Either 4 per city, like Armies or auto-produced from a building (1 per four+ turns). Up to you - I haven't had time to play a game with it yet.