Stat wise? Acid would top this but if you want to know a little bit more then you can read this explanation that I put up on summary hehe.
Summary
As for general use in pvp, I think it comes down to how you play. I am not a pvper or anything but this is just a bit of an insight to this.
Plasma is essentially a slightly faster fire with better clash rates. I believe that plasma clears one of the most status effects among the three you have chosen. Plasma seems a little underwhelming but I am not one to speak knowing I am not a very competitive pvper.
Fire is quite a well rounded magic like plasma although it is slightly slower but it deals more damage, fire has much more synergies than the likes of plasma which can be a massive advantage due to 2nd magics being added to AO.
Fire has the worst clashrates though but this problem has been alleviated by the coming of a 2nd magic this coming revamp.
Acid is the hardest hitting magic among the three through a very damaging DoT. Amongst the three you are contemplating on choosing, acid wins the most clashes out of the three magics.
I would pick acid to make it clear as you stated that you want a better magic. Funnily enough the two other magics work very well with acid so you could pick those once 2nd magics come out.
Tl;dr - Acid
Maybe refer to meta’s guide to the magics if you would want a clearer idea on how the magics work.
But finally it really depends on your playstyle.