I'm still learning LoL, though not a completely newb at this point. I play only support, mostly squishy ones, sorraka being the biggest one currently. Many guides will suggest "use these items for AD comps" and "use these items for AP comps"
However, it seems like when I tally up the champions AD champs usually out number AP champs. On bot you have an AD carry and a champ that us usually, but not always, AP; but who never scales in damage nearly as much as the carry. In jungle it seems tanky AD champs are common. Mid is usually AP, but then again top is often AD.
So it seems like most 'standard' team comps will have 3 AD (carry, jungle, and top) and only two AP (support and mid). And the carry's damage will likely be a greater risk then the support by mid to late game; I don't get to worry about rather to focus armor or MR until mid game, I have specific items like sightstone, boots and my money generator items to rush before that.
There are lots of really cool magic resist items, in fact It seems like the MR items all have cooler actives and just seem more fun; yet it seems like I'm going to often be focusing on armor with standard team comps.
I have therefor three questions. First, am I missing something in my calculations or undervaluing magic resist in these team comps?
second, how much magic resist is still worth building in a team comp like above? I shouldn't prioritize only armor and never get any MR, even if the team has more AD then AP, I assume?
Finally, how much should I prioritize scary ults into these calculations? It seems like some of the most devastating ults are AOE magic things. If a team is mostly AD, but they have a fiddlesticks and ammumu as their magic users (for some reason?) how much should I be prioritizing MR against the scary ult duo they can put out, vs worrying about the higher overall AD?