Too general a question. Depends on the site and what you want or need it to do. Varies per site.
For my store I have a affiliate plug, but of course not on the rest of my sites. Couple of my sites use the Genesis theme, so they have theme-specific plugins.
Plug-ins I use pretty much every time..
Wordfence - Security, bad file scan and reminder of needed updates.
Discus - Comment moderation and control.
Intense Debate - Shortcuts to style my post and page content
WP Fastest Cache for obvious reasons
Easy Social Share Buttons - Social sharing and social profile links
Fanciest Author Box - Better author box than Wordpress provides.
Google Analytics and dashboard widget plugins - Traffic and visitor stats.
Kraken Image Optimizer - Compress and optimize images
MCE Table Buttons - Easy tables in Wordpress.
Premium SEO Pack - Search engine optimization.
Pretty Link Lite - Shrink, track and create better looking, less promotional looking links.
Q2W3 Fixed Widget - Fixes certian widgets so they don't move as the user scrolls.
Simple Sidebars - Unlimited sidebars, can pick sidebar per page or post.
Zend Desk - Customer support
Many others that are specific per site.