Flat AD marks/quints
Best for last-hitting. Contrary to popular belief, these are not the highest-damage marks or quints for physical dps. Arpen do more damage, especially later on, to champions. However, flat AD helps early on with minions and jungle mobs, so they'll really serve you well when you decide to jungle or go top.
Flat Armor seals
Always always amazing. Great against the enemy AD bot, great against jungle mobs to keep you healthy, and great top for that oh-so-necessary survivability. Later on, if you play an AP top, you might change these up, but flat armor is glorious for almost everything.
Too useful, again. Helps mitigate damage from the enemy support, helps bring down damage from the enemy AP when you gank as a jungler, and always helps a lot late game especially since most AP carries will snag sorc boots.
Movement Speed quints are also not a bad idea anywhere, but they're expensive and a bit trickier, since you might not stay as healthy without help from teammates.