Once I saw Flash on Ezreal.
It was very annoying and got to the point where I had to move ahead of everyone else to intercept his flashes.
What's the ideal item to build when going up against a team mostly...
ap?
dps?
tank?
I normally rush to warmogs and either boots of swiftness or mecury, but I'm not sure what's best vs the champion types.
*Edit, btw I'm playing Cho.
Only rush Warmog's if you're doing exceptionally well. It doesn't provide farming capabilities, and is inferior in survival until you get a lot of charges. For a hero like Cho you don't have any problem getting charges with stun + silence + your passive so there's no harm in getting it later on.
AP - Spirit Visage (early game), Banshees (very cost effective), Force of Nature (better late)
DPS - Sunfire (the sooner you get it the sooner you can mow down creepwaves), Thornmail (the one hard counter to every single dps hero), Randuins (one of the best all around tanking items with a slow to boot)
Tank - No advice here, either you outtank them or outdps then. If you have an auto-attacking hero, yell at them to get Madred's. If there's a caster carry, Deathfire Grasp. Beyond that Armor Penetration and Magic Penetration as appropriate to your carry hero is important depending on what items they get. Generally it's not your job as a tank to deal with tanks beyond getting in their way with your huge bug ass.
Items good for nearly all situations: Aegis (awesome underused (in normal) item), Guardian's Angel (For teams with mix damage and to save your charges, also to make chasers rage)
Tip for tanking as Cho: Cho is the only tank in the game that can draw damage by virtue of being huge. Get your charges, then stand over your carry. The enemy will have a bitch of a time clicking them and you might wind up eating stuns meant for them. That or delay them from using their spells right away.