Pro's

High Damage
Scaling like no other (Crit + AD + Attack Range + Lifesteal)

Con's

Squishy
Can't handle attack speed as well as others
Weak early game

The recent changes in 11.17 buffed Senna's Crit damage, which makes IE more valuable than Rageblade.

Glacial Augment
This is Senna's standard goto-rune (unless you go tanky), since the slow helps catching up to people as well as kiting.

Magical Footwear
Movespeed feels really good on Senna, it (again) helps with kiting and catching up.
Free boots are good already, the extra 10 MS is quite handy.

Biscuit Delivery
They help to sustain through the early game and to regenerate after a bad trade or to help with the mana issues early on.

Approach Velocity
Helps you catching people + it is easy to proc the 15% MS with a RFC AA

Presence of Mind
Helps you with mana issues, this Rune alone is enough later on, for the early game you will probably need Biscuits as well.

Legend Alacrity
Since Senna converts AS at a poor rate, so you need a lot so you will feel a difference, so get as much as possible already in your runes

Bonus Runes
AS: get as much as possible
Adaptive: more damage (that's what you are supposed to build on Senna)
Armor: Standard, unless enemy team has a lot of magic damage, than you can switch this one to the MR-Rune

Flash
Is a must-have, since you are squishy and need it as a getaway or as a gap closer to secure kills.

Barrier
Is the standard since it gives you protection, it can absorb more damage than heal and it has a lower cooldown than heal.


Heal
There are only a few cases where Heal can be useful: if you're against a high poke/aggressive comp and you know that your Q-healing won't be enough and your ADC refuses to take Heal

Starting
Spectral Sickle is the standard, since you play a ranged champ and it is easy to A and land Q's or W's, Senna stacks it way faster than Steal Shoulderguards + you want to kill as little minions as possible since this increases the soul drop rate.

Two Pots are recommended for sustain (since you are weak early game), in case of an invade you can also take a control ward, but that is a rare case.

Rush RFC
You don't need to worry about boots, since you get free ones and have your E to get away or to chase.
Building RFC gives you a massive range boost, making it easy to empowered AA, then walk up a little bit and Q, since they are slowed by glacial.
This poke can be really annoying + it heals you. If you poke the ADC, the enemy Botlane will most likely not continue the trade, since the ADC already lost a lot of health without even being in range to AA you.
Also the RFC doesn't reset it's stacks when picking up souls, which makes it really easy to safely stack your passive.

Boots
You usually build Swifties post RFC, since they grant a lot of MS to help you to poke + get out.
In case of an high-AP or high-AD enemy team you can go Mercs or Stealcaps.

Standard
Mythic
Standard: Kraken Slayer
This is standard on Senna for quite some time now since it helps to actually feel a difference between her base AS and current AS + it helps with tank shredding and It is easy to proc with empowered AA + Q + AA and you can get away fast afterwards with your E.

Infinity Edge
Boosts your crit damage up to 195%, thus helps you to 3-shot people

Afterwards
Mortal Reminder for Anti-Heal
Lord Dom's in case you are playing into a lot of tanks and need Armor Pen
Collector is just a fun (but viable) item + provides Lethality so you can kill squishies even faster
GA for more survivability in case you are playing into fed Assassins or Divers

With one item, it gets extremely hard to get to you because of your insane Range.
With two Items you spike and start to do a lot of damage, especially with the Kraken slayer proc.
With thre items enemies should run before they get in RFC range.
With four Items ytou can 3-shot everyone except tanks; it's too late to run for the enemies: RFC Range -> Aproach velocity makes you faster and glacial slows them -> Q + AA -> they are dead.
With four items + Elder two autos are enough to delete someone.

You need to max Q first, it gives the most DPS + it heals you and others
Afterwards you usually max W for longer roots more often (so you can fish with your W)
But if you really need the Movespeed you can also max E second.
Important: Maxing E only increases the length of the mist and not the MS itself, so maxing E is rare.