Basically I've been playing ranked solo queue for a while now, I've been stuck at 1460-1520 for a while and I've only been playing AP champs pretty much e.g Anivia/Kassadin/Soraka/Annie (kass/aniv my mains).

So I've decided I need to switch things up, get more versatile and spread off from just plain AP as there's been situations where I've been required to tank or go DPS, so I'm wondering which is the best AD carry to play in your opinion? I'm thinking either Twitch, Ashe or Tristana and possibly Miss Fortune. Leaning alot towards Twitch though as he doesn't seem too difficult to use in team fights, just know when to enter the battle and clean house it seems, sort of like Kassadin.

Also, what are the best runes/masteries for AD Carries? I'm thinking 21/0/9 with

Marks - Armour Pen
Seals - Mregen/Lvl
Glyphs - Flat CD
Quints - Health

Possibly switching HP quints for Armour Pen quints. Thoughts? Opinions? Answers appreciated.

Marks: Arpen / AD
Seals: Per lvl MP5
Glyphs: Per lvl MP5
Quints: Arp/AD

I haven't done any Arp vs Damage calculation yet, simply due to the fact that i won't buy AD runes in the near future.
As a carry in mid, 50% of the game is decided by your early game.
Means, Harass is very important early on
More MP5 -> more harass
@ lvl 18, you'll have 28MP5 just through your runes, this should be more than sufficent for teamfights.
CDR just isn't worth it as i think, it's rather uselss early on as you just won't have enough mana to spam each CD, nor will you be able to hit your enemy with each CD.
HP Quints are strong early on but become quite weak late.
Either i'm getting focused and was in a bad position, then i'll die.
Or i was in a good position and can dominate the teamfight and survive.
100 HP don't really make a difference here.

Added: Mainly goes for Ashe/MF/Ez

Dodge or something might be better for seals depending on the carry. At least for carries that aren't very mana dependant like Trist.

Dump mana regen for more armor pen, buy manamune for infinite mana.

However champs like kog maw/twitch might benefit from attack speed aswell.

masteries are usually 21-x-x indeed, if you use ghost/teleport then get it from utlity, but oherwise the extra 9 defense points are better.

Also protip: if you get manamune and that point in defense mastery that increases hp regen per mana, you get like 50 hp regen with buffed manamune, which is pretty handy.

And for jax always go 9-21-0 for that arador point.

I would never get Manamune.
From what i've seen, Twitch works great with Critchance
Don't play your carry with teleport, except you got some kind of tactic with it and it fits the team.

Jax is not a carry.

For champion choice @ OP

Ashe offers alot of Utility (AE Slow, 100% Crit chance initiate)
Her ultimate is more than strong if used correctly.

Tristana does have alot of early burst and escape/engage mechanism (Rocket Jump+Buster Shot)
I never really got to play her, but i've seen alot of people play her well and dominate with her.

MF is quite strong early on with a very strong Harass, her ultimate Dominates teamfights from lvl 6 to 14~, her MS can come in quite handy.

Twitch is (in my opinion) not a too strong laner as he has no harass, his ganking and 1on1 abilities though are quite high, his ultimate isn't as strong in teamfights as MF's early on (my opinion) but scales much better into late.

I get magic res @ 18 on glyphs.
Casters are the bane of low hp carries.
You get a tonne of mana regen from seals alone.

Twitch is a great pubstomper but to play him right isn't easy, maybe you have a knack for him but that's a different point. He needs a little manareg but not much, he has HUGE damage, and he needs a bit more defense(tree) than the other main ad carries.

Ashe and trist both need manareg glyphs AND seals, they're also great carries but just a little more team dependant, twitch is really a rapist if played well, and against solo qeues can (most of the time) singlehandedly carry a team, as long as your team doesnt feed too bad. Ashe needs a bit of a more competent team because her ult and skills don't do the highest damage, but have the most utility and she still works really well as a carry. Tristana is just farfarmfarm until you got ur inf edge and maybe a last whisper, and then with a lizard buff +elixirs you will destroy the enemy team. Tristana is one of the best mids and can rape most other champions in mid, only a few compete. She's a great soloqeue stomp too.

This if u want pure carry. Magic resist is hardest to come by if u focus dmg as physical carry and also most dangerous dmg for carries is magic. If ur dead u cant carry much can u? Also manareg on yellow and if u have mana problems manamune solves it as already suggested and as for red and quints, armor pene is always a win choice, but also consider critical chance/damage. I personally use critical dmg quints on ashe and works wonders. Def not flat hp as already said its useless past few first levels.