IMP medium-range ninja - Throwing / Auto Weapons
Night Ops (Expert) seems to be the most popular choice for the IMP merc. The main appeal of Night Ops is that it makes you a better sniper by extending your range of vision at night. That it also reduces the need for sleep is just icing on the cake (so does Martial Arts). AIM has three great Night Ops snipers (Lynx, Scope, Raven), and night vision can be extended with equipment (+4 from UV Goggles).
But when you're fighting at night, you're spending most of the time controlling your ninja, as you stealthily explore the area and eliminate the easy targets. Your ninja's experience level therefore skyrockets, which means you end up paying a fortune to keep someone like Blood employed.
Blood's description on AIM reads "his ability to fling a combat knife into an enemy's neck is a sight to behold: it seems to come out of nowhere, sails an impossibly large distance, then slices through its target with unerring accuracy". Though "unerring" accuracy is an exaggeration, you'd be right to take this description as a hint that throwing knives are unrealistically good in this game. They have a range of 15 tiles, and instantly kill most unaware targets.
A silenced SMG has a range of 20 tiles, and can be used when throwing knives are not cutting it.
Throwing and Auto Weapons also happen to be useful for increasing your damage output in general (assault rifles, grenades).
Why not Night Ops for a ninja? Because seeing your target from further away can actually be counter-productive, because it switches the game into combat mode while you are still out of effective weapon range.
Why not Stealth? Because when killing unaware targets at medium range, it's enough to stay low and and move stealthily. Stealth is of greater value to melee martial artists.
Why not a melee martial artist? Because this style of combat is too dangerous unless two ninjas are working in unison (else a single failure results in grievous injury). Also, the game has a contrived stealing mechanic--you can steal items from opponents who have been knocked out but are not yet dying. These items do not drop when the opponent dies, which makes no real-world sense. So if you're going with a duo of melee ninjas, you're putting yourself into a situation of profiting from what feels like an exploit, which can spoil a game. It gets worse if you start to feel that you're wasting resources unless you rob everyone before killing them, because then you've doomed yourself to difficult combat (and then what's the point of all that money?).