Yep Same+1 for Avast - been using it for years and never noticed any drop in performance.
ESET is very good and also non intrusive. It's been worth the price for me.
Moved from Avast Free to Bit Defender Free because it was less intrusive. However(!), I couldn't get it to work on my other 2 PC's, it would just fail to initialise over and over.
I've been using Malwarebytes since around 2004 and never had a single problem. I used to regularly have the same few people round my house with their laptops, due to spyware, ad-ware and viruses. After converting them over to Malwarebytes I no longer have those annoying regular visits.