How you build your champ matters almost as much as your team comp.
I've seen countless teams throw away their chances of winning because their only tanky champ wanted to be AP carry #3 instead of building tank. Or their only ADC builds poke/lethality into 2-3 tanky enemies instead of DPS. Or 3+ players all wanted to play an ADC. Or nobody builds grievous into a heal comp or serpents fang into a shield comp. The list goes on and on.
I got a bunch correct in a row by just selecting the team with more tanks until I picked a team with Tahm/Voli/MF that all decided to build full AP.