For me, if the champion's abilities are better utilized by me using it, like blade, I would put it on attack. And champions that don't require full combos or good parry timing, as well as those whose abilities really complement the nodes, I will put on defense. Nightcrawler and mephisto are great examples.
If a certain champion performs well for you in quests, it might be worth trying them for attack in war. If a certain champion always gives you trouble in quest when going against them, try them out on a few different nodes on defense in war.
It comes down to what specific abilities they have that are annoying and might cause a potential death and also any node interactions that might enhance that possibility.
Basically if you're a decent player and there's a champ in EQ or Story Q that gives you a headache (you die or almost die) then there's a good chance others struggle with that champ as well. And if there's a node in the war that makes that champ more likely to cause a death, then all the better.
For instance, Nightcrawler is annoying for most people under normal circumstances. He gets much more annoying if Stun Immune or his abilities were enhanced to proc more often. Since Stun Immune and Enhanced Abilities exist in the war, and since he's already annoying on his own, he makes a good defender. Same with champs like Black Panther Civil War, Mordo, Magik, Hyperion, the new Iron Man, Medusa - again any champ that is difficult in arena/duel will be just that much more difficult on the right node in war.
Hope that helps
Also consider special movies that would be terrible if unblockable because there are unblockable SP1 and SP2 nodes in war, assuming you're the right war tier. So an average champ might become a viable war defender (like Cyclops) if put on an unblockable SP1 node.
Also certain high-regen or high HP champs can make good war defenders on the right node if you can get the enemy to time out without killing him - i.e. 6* Juggs on bleed immune/poison immune node. So many HP that someone might time out without being able to apply debuffs.
One final point to consider is "chip damage" not every champ will necessarily get a kill, but like, say, Electro, he causes unavoidable damage (unless the person brought BW) and so while he won't get a kill, he will weaken the opponent, forcing them to either spend pots or risk a fight further down the line against a harder champ that will for sure get a kill because the "chip damage" champs took away part of their health bar.
Comments
If a champ has a lot of utility/attack/ways to counter opposing abilities, it's usually good for attack.
Don't use Hulkbuster for anything, don't even bother upgrading him, period.
Basically if you're a decent player and there's a champ in EQ or Story Q that gives you a headache (you die or almost die) then there's a good chance others struggle with that champ as well. And if there's a node in the war that makes that champ more likely to cause a death, then all the better.
For instance, Nightcrawler is annoying for most people under normal circumstances. He gets much more annoying if Stun Immune or his abilities were enhanced to proc more often. Since Stun Immune and Enhanced Abilities exist in the war, and since he's already annoying on his own, he makes a good defender. Same with champs like Black Panther Civil War, Mordo, Magik, Hyperion, the new Iron Man, Medusa - again any champ that is difficult in arena/duel will be just that much more difficult on the right node in war.
Hope that helps
Also certain high-regen or high HP champs can make good war defenders on the right node if you can get the enemy to time out without killing him - i.e. 6* Juggs on bleed immune/poison immune node. So many HP that someone might time out without being able to apply debuffs.