My own PERSONAL preference is... In this order...
-Statikk Shiv
-Infinity Edge
-Bloodrthirster OR Blade of the Ruined King BASED on if they have more than one HEALTH tank.
-Two Defensive items, typically a Maw of Malmortius for AP heavy teams and/or a Deadman's Plate (Synergizes quite well with him) and/or Guardian Angel.
Now, I say BT or BotRK because with an IE, you will do 225% of your AA's on every Q and AA. This means a BT's 80 AD totals about 180 damage. BotRK's 30 AD becomes 67.5 damage, we'll say 68 for round numbers. That's a difference of 132 base damage that the BotRK's passive has to make up. So to find out the bare minimum of health, we'll do 132=.08x, with x being the health, which means BT starts doing more as soon as you get to 1650 or less. This is not taking into effect that your Q only applies the BotRK's passive on the FIRST target hit, not on any of the others, which is why your Q will give the same LS no matter how many things you hit with it and is also why LS is even noticeable in a 1v1 situation with his Q (It's AoE, but because it only applies to one target, the LS is not split into 3rds and it was done like this so Hydra would not be broken on him.).
I should note that you need to run 10% crit chance in your runes (Prior to passive) to make it to where you can have 100% crit chance after the IE. This makes your early game considerably weaker, but Yas's early already isn't the best and he's more of a mid-late game carry unless fed anyways and you should aim to be a mid to late game carry when playing him.
Boots are completely based on personal preference. With just the Shiv and AS from Masteries, you'll have your Q at it's max since it scales .6% CDR per 1% AS, down to a maximum of 66%, which means you need 100% AS. You'll have 40% from Shiv, 5% from masteries, and you get 54% by max level just from leveling, so Zerker's could be good early if you feel you really need the CDR on your Q, but it's not really necessary. When against a team with CC, Merc Treads are without a doubt the best boots, especially when playing Mid lane Yasuo.
Also, to answer your question about Hydra, BT, and BotRK... Well, BotRK will do a substantial amount of damage to tanky targets, but due to you having more than enough AS with just a Shiv and levels alone for your Q, it's not a big enough DPS boost if the enemies are a bit squishier, so we have to compare BT and Hydra since BotRK will always be the best bet for health tanks. Hydra has 12% LS and it'll proc the AoE. This AoE isn't a whole lot of damage since it doesn't affect your current target, but the explosion can be useful when used in conjuction with your Q, aiming for the animation resets. Typically, you do a lot of damage as is, which is why this item isn't so popular on him on top of the fact that his Q only procs on hits on the FIRST target it hits, meaning if it has to hit a tank prior to getting to the enemy in the back you want to ult onto with that tornado, it might not be best. It does add for a lot of AoE damage if you can survive in a team fight since your Q by itself is AoE and the Hydra can add to it. I personally prefer a BT over Hydra just because the shield is easy to stack up with the high crit damage from Q and AA's, which helps you stay alive on top of the 8% more Life Steal.