It mostly depends which character/build you play with.
The classic boosts on elemental damage, AAS, ASE or URad are generally good in all situations.
- “Stacking 1%” is beneficial if you can reliably stack more than 100 bullets without missing 1.
- “Stacking 5%” is beneficial if you can chain more than 20 kills in 25 secs.
- “500% elemental nova on crit” is great when you crit and adapt elements to target
- “300% damage on target above 90%” works only on first shot. It s great if you need only 1 shot.
Others are situational, like reduced damage or AAS novas, these are great on guns with situational effects, ie. Vanquisher smg, that you’d use for moving more than shooting anyway.
Some effects would just never fit into your playstyle, like if you play only Zane, AAS would always be more desirable than ASE.
For combinations, keep in mind Guardian ranks add 5% damage per element, so mixing elements should have better results. Cryo and Rad have less penalty against non-matched target.
Sidenote: I happened to try 500% crit nova on a double-barrel nuclear Jericho in mortar mode (rocket launcher), with blastmaster Moze. It works extremely well, to the point nothing would survive, including Moze. If you search new ways to blast yourself, you may also try Complex root sniper.