I agree with William. You must choose your theme based on your goals. Once you have your prefered theme, you have a huge list of plugins out there that can help you in your SEO tasks, but let me give you my favorites:
-WordPress SEO by Yoast (Andy and William are correct, it's the best): it lets you do a great on-page optimization, e.g. titles, meta descriptions, rel canonical, robots, etc.
-XML-Sitemaps: although Yoast plugin gives you the possibility of auto-generating a sitemap of your website, lately presented some operational problems. So, the best option is XML-Sitemap, where you can generate and send to search engines both sitemaps and compressed sitemaps.
-W3 Total Cache: SEO is not just about on-page optimization and linkbuilding, but website performance too. You need to pay attention to how your site appears to users: load speed, browser cache, javascript and css minifying, etc. This plugin lets you control all these variables (Warning: be careful, you must know what you do).
-PageNavi: this fantastic plugin helps you with the pagination of your website, one task that sometimes can harm your SEO efforts.
Apart from these plugins, there are themes that come pre-configured to support the optimization of your website. You need to use these frameworks and work with child themes:
-Thematic: made by Automattic, the WordPress Team.
-Genesis: by StudioPress.
-Canvas: by Woothemes; is not a framework itself, but comes with a bunch of SEO features; recently announced that the SEO features would be replaced by Yoast plugin.
Hope to be helpful.
Sergio.