I've been trying to get better at this. Usually I look at who the enemy support is, and who my adc is.
I rarely pick a mage into a tank. It can work, but I'd generally rather not.
I pick sustain into sustain (and I generally like sustain champs better anyway). Playing against a sustain lane when you have none can be really difficult.
Nami isn't bad with anyone. She's an excellent pick all around. Her w is basically the easiest harass in the game and it heals you, too. Bubbles take some time to learn to hit, but are really powerful. I love her ult. Use it to engage or disengage as you please, or even execute that pesky adc who got away. I even stole a dragon with a Nami ult once. Her e is great with adc's that rely on autoattacks. My buddy mains Lucian and we play Nami/Lucian all the time because her e works so well with his passive. She has so much cc that she's good for and against almost any comp. Nami is also a NOTORIOUS lane bully. Her poke is mad. Max w and cast it on opponents as often as possible.
I don't play Janna. I imagine she's good for adc's with no escapes (Ashe, MF) and good against dive/all in comps (ult and watch them scatter).
Thresh is only good if you can land hooks. He's very good with all-in comps, but not great against sustain lanes (Unless you're the rare Thresh that can make a Relic Shield work, but there aren't many of those.) TBH I don't understand why Thresh is that great. He really can't do much unless he hooks you, and anyone with half a brain has good enough positioning to not get hit by those. Sometimes Threshes are silly and hook the wrong person, too. I play support Naut for lols and if Thresh hooks a Naut, that Thresh and his adc are dead meat.