which adc should I buy?

Ok, for solo queue, stick with either Tristana or Vayne. I would more likely prefer you play Tristana. Ill give you reasons too.

1. She has a great escape with her Rocket Jump.
2. When pushing waves/towers, most of the times teams do not like to group up. Well Tristana's AAs' combined with her E on tower can destroy towers within seconds. if a Tristana is left alone split-pushing, then that is a serious threat to the enemy team. She can push waves easily if you decide to go statik shiv on her instead of Phantom Dancer.
3. She has a good gap--closer with her E/Ult.
4. Her E passive is fantastic for early game farming and another reason she is very good for pushing lanes late game.

All in all, stick with Tristana mate. Listen to my advice and go spend your IP on some other good things. You wont regret my advice either. Trist is one of the strongest adc's in the game right now.

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Draaaven

It doesn't matter who you buy. you will eventually get enough IP to get another one. just pick someone that looks cool.

How do you like to play...if you want a burst ADC look at someone more like graves or draven...if you like sustained damage go more cait or vayne...I personally like MF but its in how you play they can all be strong if they fit your play style

Draven if good, twitch if good, vayne if very meh but wanna be a pussy, tristana if you wanna feed but still take towers, graves for dat cigar smoking dmg early or urgod bcz he is so god

I'd go with Cait or Ashe. Tristana is not in a great state right now, so I wouldn't go for her if I were you. Jinx pushes just as fast and does everything else better. Jinx and Ashe are both in a pretty good spot right now, though they lack mobility. Cait is just relatively safe because of her range and doesn't really get countered very hard.

Draven has amazing damage if you can consistently catch his axes, and his ult can really ruin someone's day. Ashe's rework was very kind to her, and now she's a very strong pick, especially with a global ult and her reworked Q.

I'd also suggest Urgot because of his awesome damage potential, but I think that he works better with a tanky build than as a straight-up ADC.

Ashe, ezreal, corki

How do you like to play? (Passive, lots of poke, long range, AA based, or caster) If you like passive and just wanna farm to late game Cait or Vayne is good For poke I`d say Cait, Kallista or Ez or Kog Long range is Cait and Varus AA based is usually Jinx, Vayne and Kallista Caster would be like Ezreal, Kog, or Graves