Manamune is cheaper + Harass with Q early game.
EDIT: Also, rather than saying manamune sucks, suggest other items so I can try it out.
Basic AD Carry build.
Start with Dorans Blade if you have a support, Boots 3pots if not
Now theres 2 ways you can go from here. You can go for the "Early damage" build or the "Sustain/Farming" Build"
For Early damage buy 2-3 Dorans and Berseker Greaves. Then you want to build IE or BloodThirster.
For Sustain build you want to go Doran's at start then you buy wriggles boots and then phantom dancer.
After this its the basic IE / Bloodthirster ---> Last whisper ----> Then some defensive item like Guardians Angel
Trinity force doesnt really work for any other ranged ad than ezreal but hes stuffty earlygame champion anyways so no dont buy this item.
Full Build
Doran's--->tier1 boots--->Wriggles--->upgrade boots--->PhantomDancer--->BT/IE--->LastWhisper--->GuardiansAngel
if you reach this point sell wriggles and buy another bt/ie
[sell dorans blade if you need inventoryspace or quick 200g when you feel like it]