Core:
-If vs 0-2 tanks, first item Collector to KS and get fed.
-If vs 0-1 tanks, second item Shieldbow for survivability.
-If vs 2 tanks, second item Kraken for tank shred.
-If vs 3+ tanks, first item Kraken, second item BotRK, third item Hurricane.

Starting Items:
-If vs 0-2 tanks, start Dirk and Dagger OR Boots. Pick Boots when on a bursty, low attack speed hero like Graves, otherwise Dagger for higher DPS and better CSing.
-If vs 3+ tanks, start Noonquiver.

Note re Collector first item:
In Rift bot lane you're just competing with support+jungler/roamer for early kill secures, so Collector is pointless. In ARAM, you're competing with 4 other thirsty players, so Collector's value is infinitely higher.

Recent example of the Collector 1 item spike:

Flash + Heal is core. Barrier is a strong alternative to Heal on immobile, aggressive carries such as Twitch.

Dark Harvest, Coup de Grace and Collector will get you fed. If vs low-tank, high-damage comps, Taste of Blood and Ravenous Hunter will synergise well with your Shieldbow to keep you alive/topped up. If vs multiple-tank, low-damage comps, you'll need the healing from Taste of Blood and Ravenous Hunter to stay topped, as you won't have almost any lifesteal from items until late game. Triumph will maximise early KS gold income and keep you alive in late game teamfights.

-Take early 1 for 1 trades, as they're worth the death to speed up your Collector purchase. Flash forward to secure them if need be. Start playing safer only once you've completed an item or so. Your 2-2 KD can then turn into 10-3, 20-4 etc. You want the gold on the carry, so early in the game it's worth it for you to Flash in on 50% HP to KS an enemy at 5% HP.

-Always look to be autoing something. If enemy champs are slightly out of auto range, auto a creep for gold and lifesteal. Don't waste time.

-Always position forward/aggressively. Not necessarily right at the front, but always close behind/to the side of your tank/engager. This will allow you to capitalise on plays, secure kills, and net you high CS.

-When a creep wave is approaching, step forward, as you can usually get an auto or so off on it for free, before the enemy team reacts to you stepping up.

-Have a plan/know your role in the teamfight. Are you kiting front to back, or are you Flashing deep to kill their 1 fed player? What have you built?... If Kraken, you'll almost always fight front to back. If you've built Shieldbow, you have more freedom to fight front to back, or Flash deep to reach a priority target, as the situation requires. Will you position on the left side of the lane? The right side? How will you reach your desired position before committing to the fight?

-Know the threats. Which enemy skills will you burn Flash to avoid? Which will you sidestep? Which will you soak to trade and then lifesteal after? Do you need a QSS (don't build this unless absolutely necessary)? Should you sell boots for 6th item Zhonya's?

-Pick your moment and fully commit. Has the enemy assassin ulted already? Has their tank blown their CC on somebody else? Once a few key CDs are blown, step up aggressively and don't stop autoing.

-If you've committed to your positioning and realise that you're going to die, don't try to run. Get as many autos off as you can before you go down. Often in this situation, you may even be able to Flash deeper and secure a kill just before you die... Low health enemies love to step back into your auto range and try to KS you when you're about to die.

-Your summoner Heal can be used in a variety of ways. You can use it to sprint forward and secure kills. You can use it to sprint backward and kite. You can use it on CD to stay topped early.

-Always be APMing. It's easier to dodge things, and therefore position more aggressively, when you're constantly in APM mindset.

-If Champ Select doesn't roll you any ADCs, and your comp needs AD, anything with a ranged auto will do!...
e.g.