avast! Free, plain and simple. They've been very good before and with version 17, they've gained several key technologies from AVG acquisition. It's a really powerful solution with very little limitations in free version and with rather minimal "nags". And from my experience, they have one of the best supports through forums because there are regulars who are in contact with devs and devs also participate there so issues get fixed really fast if they can be correctly identified and reproduced.
Comodo is not bad from the concept side of things. The idea behind their system is brilliant. Too bad it was then "visualized" into an actual program by a baboon, their forums are overcrowded by the most hardcore fanboys I've ever seen, criticizing them will get you banned instantly (almost anyone I ask says they got banned there for simply questioning something), they don't ever fix their s**t unless someone of fanboys somehow picks it up and pushes the thing forward with the devs, they endlessly promise things and never realize them. Their Valkyrie analysis system for example in CCAV, they still keep on promising how it'll "get better" just now and that "just now" never happened even after 1 year and a half. They could be the best solution in the world if they got their heads out of their rears, but they stubbornly refuse to do so. Go figure. It's alright if you don't mind declaring what's clean and what's malicious yourself, because their auto system is garbage (yes, Valkyrie as well, because it's slow and often classifies things wrong). And if you'll ever have problems or some wish, good luck fixing that. I wouldn't even bother because they are more concerned over the format of bug report than by the bug itself...
Good alternative is also Bitdefender Free. It's now compatible with Win10 and sports basically identical protection engine as paid versions. Has almost no settings, but it's easy to use and has good protection. Requires creation of account to use. It's free but many people don't like that...