Well, on the hardest difficulty setting on TW2 I used Quen initially since it felt almost like having double your health bar
It's not my favourite though.
Overall, over the two games, it's a close run thing between Aard and Igni. Not very sophisticated I know but 'force pushing' people on their ass or simply setting them ablaze is pretty satisfying.
A tough choice but I eventually voted for Igni. I loved using it in TW1 since when people were ignited they took silly amounts of extra damage when using certain sword styles iirc.
I'm definitely gonna try and experiment a lot more in this game though, especially if you can re-spec. It's a long game, more so if I'm gonna go through twice, so I want to discover as many 'flavours' as possible; after all the effort CDPR put in, it would be a shame not to.