Always gotta go with Yoast SEO. Mainly just for the custom title and meta description features. Occasionally set up the OpenGraph stuff.
I like Spam Destroyer for blocking botspam, which I’ve found to be extremely effective. Akismet is great too, but I’d rather not waste my API key on a site that’s not specifically targeted by more sophisticated spammers yet. Even brand new sites with absolutely no real people traffic are immediately hammered. I find that amazing.
I try to keep my plugin footprint light, but depending on the site, I might install a few of these:
Subscribe to Comments. The old school Mark Jaquith version. Essential for keeping commenters engaged. I considered using Jetpack for this, while they handle the email delivery…I think some audiences would find the WordPress.com branding confusing. I’d consider it (even for a premium hint hint) if they let us customize the logo or something. I personally feel sketchy sending emails from my own server, would rather somebody else handle email delivery.
MailChimp for WP. I’ve found this to be better than the “official” MailChimp plugin, which I use it for my newsletters.
I’ve never used it before, since my WordPress sites tend to be pretty light on images, but I would definitely consider using Jetpack’s Photon module as a free image CDN. I think that’s an insanely valuable feature.