Eh, it really depends on both team comps. In solo Queue, it may often be better to put someone with a crazy strong early game like Renekton in the bot lane instead of an ADC to shut the other team's ADC down hard and snowball to a win in the mid game, but generally that route should only be taken if the other team has already picked an ADC that can't handle it like Jinx or Twitch.
Against point and click "fuck that guy" champions like Nautilus and Vi, you really want someone with good range and self peel like Cait, or to play someone that can wait until the fight is well underway before going in and nuking the other team with high short-duration steroids/burst like Twitch.
Knowing when to go in and not getting isolated is what will keep you relevant in the mid-game. If you go in too early, you will be dived, and if you go in too late, your team will have already lost the fight. Sometimes it's better not to fight, because your team made a stupid call, and you going into the fight won't make the difference in getting kills, or farming a few waves would end up giving the team more gold overall than any kills would have been worth, and denies the enemy team a kill (I know this is contrary to what most people say, and for any other role, I would say otherwise, but as ADC, farming instead of joining that fight has often been more beneficial than joining bad calls and has turned games around).
Really though, games tend to come down to which team has more CC/mobility. If you can get a team with more CC and a good mid/ADC, you will usually win the fights and snowball the game, because solo queue tends to revolve around team fights; as for mobility, the team with more mobility usually dictates the pace of the game, because they can rotate faster to be at objectives and will generally be better at forcing or disengaging fights and keeping their lanes pushing.
For ADCs in general, timing when you go into the fight and positioning is everything; if you can sit near a bush or over a wall, that's ideal. Long ranged ADCs such as Cait/Jinx/Trist have an easier time with positioning because of their long range, and are what I recommend as go-to picks if it is early in the draft because they can be rather self-sufficient and don't need a team built around them, and don't have any exceptionally bad lane match-ups. Graves is another alternative because he has good burst, which lets him play more like a mid-ranged mage that autos a lot than an ADC.
IMO, you should never be going for a defensive item until after at least first item and Zeal; and even then only part of one, like just the sash to answer Zed/Malz (Though IMO, QSS and Zhonya's shouldn't exist; items countering entire champions is pretty poor design), though usually I will just stick with the standard damage for the first 3 items and then choose the 4th depending on how the game is going.
BT first is probably the best build option if you are being targeted a lot or the other team has strong poke; 20% lifesteal is a lot, and the shield is great for burst mitigation.
