Buy only legendary and 6 star runes with good stats and substats, check out next paragraph for stat review.
Look for %ATT, %Crit dmg and %Crit rate as substats in all runes and for even runes look at the following main stats suggestions:
|2||SPD||Can be good for some speed based damage dealers|
|2||%ATK||Optimal for bombers, good for damage dealers|
|4||%ATK||Optimal for bombers, good for damage dealers|
|4||%Crit dmg||Can be good for some damage dealers|
|6||%ATK||Optimal for bombers, good for damage dealers|
Generally speaking it depends from the crit rate you can achieve, but the difference is situational. Rage Runes can be better than Fatal runes when you have at least 65% Crit Rate for generic nukers. For Bombers always prefer fatal runes since bombs don’t crit.
Fatal runes are really useful on monsters like: bombers , nukers , attack based healers
Here is also a couple of monster with the appropriate link to the detailed stats and review