Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I've seen a lot of Caitlyn builds on here and most of them are good, but I've had lots of success with this one, so I thought I'd share it. The last few patches have been kind to Caitlyn and I feel that her late game presence is much scarier than before. She is my favorite of all the AD heroes in the game because of how aggressive you can play her at mid. This is my first guide, so any comments or suggestions are welcome.
I chose per level runes because by level 6, you will meet or exceed the benefit of the flat mana and cooldown runes. Of course, if you prefer flat runes, those work as well. I would not suggest getting attack speed runes, but if you must, the attack speed runes can replace the cooldown reduction glyphs.
Your skill priority should be: R,Q,E,W.
Max Q first, putting one point in each spell by level 3. Obviously, put a point in ult whenever possible, then max 90 Caliber Net next. Max Snap Trap last.
I chose Teleport and Ghost as spells for Caitlyn. Teleport is quite helpful for going back to buy or heal, but not putting your turret at too much risk. Teleport is very useful mid-late game for defending minion waves or catching up with your team. Ghost is pretty self-explanatory, it helps you get away or catch up. Don't pick Flash (unless you're playing 3v3s), that's what 90 Caliber Net is for.
Follow the purchase order. If you go Doran's first, sell your Doran's when you build Bloodthirster.
If you see HP stacking, buy Madred's Bloodrazor
If you see armor stacking, buy Black Cleaver
Your core items are Infinity Edge and Bloodthirster. Feel free to change around the last three items if they don't fit the enemy team.
Caitlyn can "dual-channel" certain abilities. When Caitlyn channels her Q spell, she can also cast her E spell. This allows Caitlyn to cast her Q, then cast her E behind her, allowing her to make up for lost ground during the Q spell channel time. If Caitlyn is being chased, she can cast her Q, then cast her E spell in front of her, damaging and slowing her foe.
Credit goes to Girlstar for discovery.