Ranged Poison should be your go-to way to take out lone, risky targets. It can make a lot of the optional objectives a lot easier, so you could try to keep it fully stocked at all times.
The Wrist Gun is similar, but a lot louder. The ammunition is a little cheaper, so make use of it regularly to thin out enemy ranks in large group fights.
Both of these ammunition types can be looted from guards regularly.
The main utility of Throwing Knives is in that you can charge them up, to take out three soft targets at once. In fact, doing this is a requirement for a Trophy/Achievement!
The Crossbow is possibly the best piece of equipment to buy in the game. Although it’s expensive, it makes tricky sections of the game like the Yerebatan Cistern a lot easier, and it’s even an effective melee weapon!