With the Cybex Balios S, any Cybex brand car seat will be compatible - I personally had the Aton Q Plus, so I can, even if maybe with mixed feelings, recommend the actual Cybex Aton M or Aton 5 for sure, at least for safety reasons and good car seat ADAC testing.
I was, however, much happier with Maxi-Cosi car seats (we had a Pebble) because the seat surface seemed more comfortable for my son, his head was more secure in that position, and the fabrics were much softer so he wasn't as sweaty as in the Cybex. Of course, this is a subjective opinion and someone else could say something else. I also had a different Maxi-Cosi's and Cybex's and still, a Maxi-Cosi is my own choice for higher group car seats as well.
Next, I will also mention the Joie brand. Joie car seats have the same attachment system as Cybex and Maxi-Cosi, so all mentioned here will fit your adaptors, while Joie are more affordable and also well tested in safety tests. So if the budget plays a role here, Joie Gemm or i-Gemm are absolutely a good while a cheaper option for you.
Kiddy also has fantastic, but MUCH more expensive capsules with great safety tests, so, if the opposite is true and you don't look at the price at all, I would go for a Kiddy 0+ car seat if the money wouldn't be a problem ;-)
So, a Maxi-Cosi is my best value for money suggestion, a Joie for a budget option, and a Kiddy for a luxury option 😉 Hope I helped 😊