I find Aery is almost always better than Comet, unless your primary poke has a slow or some other hard CC attached to it. Examples: Lux, Velkoz, etc. Otherwise, you'll generally be picking between Aery and Electrocute. Poke or burst.
One of the best generic secondary paths in the new system is Precision, taking Triumph and Coup de Grace. If you ever don't know what to take, you can't go wrong with it.
This is my generic poke mage page: http://na.leagueoflegends.com/en/featured/preseason-update/8200-8214?build=20100016
For heal/shield supports, I go Resolve secondary with Font of Life and Revitalize. It's like having three keystones in the old system.
Generic assassin page: http://na.leagueoflegends.com/en/featured/preseason-update/8100-8112?build=10222175
The preset is good, too, of course. You can trade Absolute Focus for pretty much anything else there, depending on what you want your focus to be. I feel like assassins, while pigeonholed in their primary tree, have a lot of options for secondary. Also, you can pick another one of the vision oriented ones from the red tree if you prefer the utility, I just chose Eyeball Collection because it fits in with how an assassin wants to snowball.
The ADC preset is already pretty good. I'd consider changing Coup de Grace on it to Cut Down if against a lot of tanks, and Celerity to either Transcendence or Manaflow Band if I was playing a spammy ability-based ADC.
I haven't looked much at tank options, so can't help there.