He works the best in top lane as if you pick him mid, you need your team to pick an ap champion, which obviously rarely your team will accomodate your pick unlesss you're playing with a jungler/top laner.

Top lane Yasuo can simply farm, out scale and get his items. Certain matchups like tryndamere, darius, urgot, rushing a frozen mallet after your zeal isn't a bad idea as it allows you to kite them by spamming your q as well as making you a lot harder to kill.

E deals 10 damage per point. 50% increase gives you 20 per point. Q gives 20 per point and doesn't have a lock out window thus you win extended trades. E doesn't crit, q critically strikes so with just a zeal, you're dealing more damage per point in q than e.

Let's use an example.

Yasuo gets 70 AD from runes + doran's blade which is the common start.

Steel tempest your q deals 20 damage + 100% of your TOTAL ad. This translates to 90 damage per cast of q on around a 3.8 second cd.

Yasuo e scales with BONUS AD! It deals 60+20% bonus ad

Meaning at rank 1 it deals around 62 damage, now if you have it stacked it will deal 50% extra damage, this translates to around 93 damage level 1.

Now let's level up the abilities.

E deals 10 extra damage if I put a point into it. So, now it deals 72 damage, although we gain AD from levels, this scales on ad from items + runes, so damage scaling isn't changed. Thus deals 108 damage on a longer cooldown at rank 2

Q now gets 20 extra damage, so now that alone does 110 damage without even critting. But, we also level'd up, so now his 100% AD ratio is effected, so now he deals 3 extra damage so, he now deals 113 damage per cast of q which has a lower cooldown than e. Once you get critical strike (Which is common after first back usually around level 6-7) you'll be dealing much more than per point of q, compared to e, and obviously due to you leveling up, Steel Tempest goes on a lower cooldown as well as buying items like Zeal thus translates to MUCH more damage. Also you naturally gain AD thus more damage per cast of q.

The reason you saw people max e in previous patches is because his e got hard nerfed recently, it used to overall do more damage than q in bursty trades, but now it does less damage than your q per rank.

Whoops, i accidently gave the e a 50% ad ratio lul. So it's even less damage.

The e got nerfed quite hard, and only gives 10 points per point, 50% extra damage when stacked equals 15 per point, q gives 20 and can crit. 20x1.8 = 36 extra damage per point if you crit, more damage = higher ratio. Q has a lower cooldown, thus you can do higher dps. Yasuo damage has been nerfed overall so.. You need all the damage you can get.