Back in the Witcher 1/2 the Signs were incredibly unbalanced. Witcher 1 Aard was undoubtedly the go to sign for 1v1 fights. Meanwhile, Igni pretty much was ridiculously overpowered once you invested talents into it. Yrden ended up useless, Quen wasn't that useful and Axii I never saw a use for either. Igni just out damaged them too much and hit way too many enemies. Meanwhile, Aard just set up instant death kills and knocked enemies down. Witcher 2 came along and improved the sign balance a little fortunately. Igni got severely nerfed, but unfortunately arguably too much so to the point where using it isn't worth it anymore. Meanwhile, Aard got incredibly overpowered by being able to hit like 3 people at once, knock them back and even set up instant death kills. Quen was too good too and pretty much was the main sign anyone used for boss fights since it allows you to take one hit for free. Yrden got a lot better in Witcher 2 and that was good. Unfortunately though, in the end it was ultimately that Axii was relatively useless and so was Igni. Quen was too much of the go to sign for boss fights and Aard was the main sign for fighting enemies. What has been addressed to adjust the balancing of signs in Witcher 3 CDPR?
This is pretty much how I approached signs in The Witcher 1:
Igni. Igni, Igni, Igni, Igni, Igni
I didn't enjoy the combat so I just cheesed as much as I could
In the Witcher 2 though, this is how I approached signs:
Igni is good at first for finishing single opponents or stunning them fast. Later on it manages to deal not only a decent amount of damage, BUT it can also hit groups of people. Depending on how much Vigor you had and what potions you were using, Igni could deal with a group of measely humans in maybe 2-3 casts? So it not only became a good damage dealing sign, but also a good way to crowd control.
Aard was also a good crowd control spell and useful for finishers. Also, Aard+Cliff=$$$$$$$. It's also a pretty good spell for individual fights.
Quen is overrated. People say that it's really the only sign to use, but I disagree. Cheesing a fight with Quen takes longer than using Geralt's entire arsenal and screws you over with no Vigor regen. Investing points into the Shocking abaility of Quen is very useful for stunning multiple enemies, but it doesn't deal much damage unless you have The Human Torch Quen ability from the swamp in Chapter 1. I like to use Quen when I know I'm going to be exposed and facing multiple enemies like in the Harpy Lair.
Yrden was the only spell that I wasn't too much of a fan of. I didn't use it that often because Axii did what Yrden did except better. It could trap multiple enemies and deal damage, but I wasn't concerned about either of those abilites also because of Axii. I think the most I used Yrden for was facing that one sorcerer, the kayran fight, and stopping enemies when I wanted to Axii.
Axii was like my special spell for special things. It was really satisfying to use and also really useful. Imagine facing a group of 4 human enemies with shields with the ability to Riposte and an upgraded Axii(Plus 50% Vitality and Damage, Ability to Enchant 3 people. Casting Axii on just one of them will distract 1 (possibly 2, but unlikely) of the enemies. That takes two enemies off of your plate already. Put down a Yrden spell, and enchant a second one. Boom 2 allies. Backstab the Yrden enemy to kill him. So now you have 2 allies killing other dudes for you and one guy is down. See the potential of Axii? It's like playing the game without putting in any effort.
This is one of the reasons why I'm excited for The Witcher 3. People complained about the combat in 2, but I fucking loved it. Gimme more pls.