It really is a roster based question as mentioned above, and there are plenty of good suggestions up there too (although I do agree with that keeping 5 entire zetas saved up is a bit excessive unless you are planning on whaling out on the latest greatest every time it appears to stay on the top of the spear).
Other zetas that singly make a huge difference are Han Solo’s Shoots First, Vandor Chewbacca, Jedi Training Rey’s Lead, Geonosian Brood Alpha’s Lead, Darth Traya’s Unique (although that one really pairs best with her lead and Darth Sion’s zeta), emperor palpatine’s lead, commander Luke Skywalker’s It Binds All Things, Chewbacca’s Loyal Friend...