To my knowledge there aren't "security" plugins per se. There are several anti-spam plugins, and the Webmaster plugin can provide some server security related information. There's also the security chapter of the install instructions, but that's about it.
Most security measures pertaining to Q2A need to be done on the web server or PHP level anyway, not at the application level. And flooding/DDoS usually can't be handled on the host itself in the first place. You need services like Cloudflare for that.
Also, asking for random suggestions of "security plugins" is not a good approach to security at all. Your first step should be identifying the threats you're facing (what do you want to protect against?), then estimate their impact and likelihood (how much damage would an incident cause? what are the chances of the incident occurring? is the risk acceptable or do you need to mitigate the threat?), then work out countermeasures for the threats you need to handle.
