My "main" mid page is 21/0/9, since I play APCs for damage, and 21 offense is almost necessary for real damage at all points of the game. Sometimes I go with 21/9/0 for manaless mids, but that's a really small category.
Runes, flat MP/5 yellows, magic pen reds, flat AP quitns. Blues could be anything, but I've come to love AP/level. Huge gains at 18, when you actually need them. Manaless, standard fair as every other lane (armor yellows, MR blues) with health/level quints and magic pen reds. Most manaless are melee, so the MR/level keeps their MR relatively competitive.
Item builds depend on the game. Always. 99% of the time, sorc shoes. 100% of the time, deathcap. Other than that, it varies. Chalice when I'm not getting blue, or tear on Cass in the exact same situation. Abyssal scepter if no one else built it, or something else if they did. At least two tanky items to help resist whoever on their team got fed. Or maybe we're going double WotA. It all depends on the game.
Never "default" your build. Every single build should vary between games. Core items are fine, but the exact same build is normally a mistake.