My favorite weapon in ACIV is the Jackdaw...
But if land only, I really liked the blowpipe.
“Force has no place where there is need of skill." Herodotus
Haha, yeah the Jackdaw is an arsenal in it's own right. Every time I sail next to a ship and let off those broadsides, I can hear an apocalyptic orchestra in my head...kinda like when Frodo gets stabbed on Weathertop by the witch king, if you know what I mean 
I'm voting other and going with: "Brutes' grenades."
Only because I got some epic kills with them including an assassin contract and a roughly 10-man kill (during the mission when you needed to take our 8 or so guards simultaneously for full sync) when some idiot lobbed his next to powder reserves where I had lured a bunch of guards.