Well Zed's ult does put you on top of the target, so if you faceroll hard enough immediately after ulting you should get the full damage out of it.
BT has 25 more AD (not 30) than Hydra, 31.25 on Zed with maxed W. Best case scenario for Zed, BT adds 143.75 damage in the duration of an ult
(2.0 shurikens, 1.6 two slashes, 1.0 ult itself) plus 37.5 more for each autoattack. Hydra gives you a 203.55 damage nuke, with no additional AD
(including runes and masteries), that scales with other AD. Hydra clearly outbursts BT.
The biggest appeal of BT is the cost difference. The lifesteal makes it a bit stronger in extended lategame fights too but BT certainly doesn't have a better build path. It's not exactly easy to save up 1550 for a BFS, Hydra builds piecemeal except for the Pickaxe which barely costs half as much. And the less you autoattack, the
better Hydra compares as the Crescent active makes up a proportionally larger amount of your damage. :P CDR also favors BT, as you cast more spells and apply the extra AD to more ratios over time, however CDR is also a bad stat on Zed because his skill usage is
incredibly constrained by Energy. Hydra is worth the cost, although honestly if the game is long enough I like both and it's better to get BT early to build its stacks. ;[
Kha'zix? Yeah, no major reason to get Hydra on him. He already clears creeps with extreme ease. BT gives more AD for his poke. Taste Their Fear has a ginormous AD ratio and very short cooldown, CDR is a great stat on him, evolved Leap resets on kill/assist. Fiora? Unique interaction on her ult makes Hydra viable but I wouldn't call it a gimme either way. She definitely appreciates the waveclear, though.
Because I'd rather discuss personal experiences and opinions in this thread rather than shit like "should I get hydra or bloodthirster on my fiora???" which are things you can already just Google and find, essentially, the right mathematically-proven answers to.
What is the mathematically correct answer to Hydra vs. BT then? You can't do a cut-and-dried DPS calculation to compare the two like you might for AD carry itemization. Players don't just stand there and let melee champions wail on them, plus there's such a wide range of champs with very low to very high AD ratios on their spells that a simplified test case would be pointless anyway.
You can't discuss your personal experiences and opinion in this thread on stuff that actually happened to you? Have you tried Hydra before? On which champions, against which champions? Did you like it? What's been going on in your League of Legends career lately? Etc. You don't have to make up a "wacky scenario that will never happen, make a list most people won't read anyway" game to goad people into posting and it just shits up the thread with pointless drivel. There's generally not much
discussion surrounding such things. I mean hey whatever, not like I own the thread but since someone else brought it into question I was just giving my opinion on the matter. I don't like them, that's why I don't participate.
The BT-Hydra stuff I typed above? Natural discussion, actually relevant to playing the game, pretty damn interesting if you ask me. There's a lot more places just to take that topic alone.