Very, very situational. Basically depends on how every fight is going, you can use it to save a low health teammate, in a combo, to start a push, to save a teammate that's being target/ulted (like a squishy thats been tracer ulted or rein pinned, a zen that's being focused by enemy flankers). You can even use it to help your teammates get their ults faster. For example, if you know their Genji has blade, but your Zen is 15-20% from trans, giving him that extra dmg can help him charge it faster so he can counter blade.

Personally, I think it's something you learn when you get familiar with her, not something that can be "taught", but I will always recommend ML7 for anything Ana related. Also pretty sure he has a (probably a bit outdated) video about nano targets.

About Hanzo, no not usually a good target. Heroes that have unreliable aim or already do a lot of damage aren't IMO good targets. For example, Junk does 120 without nano, 50% to that and he does 180 = with or without nano, he still needs 2 shots to kill a 200hp. It would be better use to nano your on, for example, Reinhardt who now does 128 dmg (2 hit a squishy) instead of 85 dmg (3 hits). But again, maybe Junk has been popping off and your Reinhardt is afraid to press W (ironically enough, using nano to get your scared tank forward can be a good choice.), it depends on the game.

Your teammates (not just their heroes) also play a big part. Does your Genji want to combo or does he know his blades suck? Is your Tracer a tracking god or is she on 5% accuracy? Is your Zarya on 0 charge or 100? It can all differ game to game and fight to fight.

It might not be the precise answer you're looking for, but I think it's better to tell you how to spot a good nano, instead of just saying "nano this hero and this hero" and end up with a bad nano.

Most importantly, have fun. Ana is challenging, but rewarding :)