The idea of this thread was started since it was asked on a different thread about something else and I think the community might have some ideas to contribute to that question on a thread all by itself to be a full and very informative discussion about it on its own thread. Because it is a good question that I'm curious as to what people think. For me to answer the question: I depends on the project. I have to really give this question some thought. Because I think what's more important than a plugin is to ensure your site's code is properly secured. And you don't need a plugin for that (or anything else for that matter). You just need to know how to go into the code of WP or a theme and do it yourself. I personally go into the coding and do what I need to do any time I forsee a problem. I could create plugins of things I code manually and share with the world, but it's something I would need to sit down and think about. In the meantime, I just help people (and myself) by going into the code and adding what they want rather than giving them another plugin to add to their list of plugins. Plugins are convenient for people who do not want to spend the time on coding their own solution. But other than that, they are not a necessity. So because it's not, it's kind of hard to say which plugin should not be missed. I'm sure others have a difference of opinion and so would like to hear from anyone willing to give their input which plugin they feel is a necessity. So what do you think?
Only one? OK. I hate it when people make lists in answer to "only one", so I'll do this in three steps... Plugin: TIEtools Why: 'Cos I wrote it. On a more serious note, I think the one I'd say is essential is Akismet. Without it, your blog gets spammed to death. If you're talking about third party plugins, and putting aside "standard" plugins (i.e. non-WP-team), I think I'd go for Contact Form 7: it's rare to have a site without a contact form, and CF7 is simply the best, easiest option to handle it. Hard choice, there are so many fundamentals!
Wordpress SEO by Yoast is one of em. The other one I'd recommend is Disqus Commenting. Roll with those two in addition to the ones said above and I think that's a pretty good base setup.
Limit Login Attempts There are always people and computers who try to login to your site by guessing your username and password. I've had a couple of these automated attacks on one of my sites. This plugin helps to lock down the IP for a certain period of time after failed login and also send an email to the site administrator about the same..
Askimet is a definite must, which is why it's already installed when you install Wordpress. Another one I think is Wordpress SEO by Yoast.
I'll say a good back up plugin. Anything can be adjusted, configured, analysed again but if you loose your database and even files that contain customization... it will be headache and will take quite some time to get up and running again. BackWPup or BackUpWordPress may be.