well it depends on what purpose, in traffic i suggest google webmasters toolkit and services(free) you may also use paid for faster result. semrush and some firefox addons is also a good bet.
The best depends upon one's own choice. For keyword research Google Keywords tools can a good choice and for competitors analysis you can look through the KeywordSpy or OpenSiteExplorer.
Sitetrail.com, Google Analytics, Google Adwords Keyword Tool, SEOBook toolbar, SEOquake, SEO meta inspector, Google Webmaster Tools, Woorank, Whoisdomaintools, Backlinkwatch, InformEnter are best and widely used seo tools. According to me, Sitetrail is best SEO website evaluation tool available on internet for SEO. SiteTrail provides the ultimate all-in-one free website analysis tool.
Depending on the purpose you can select any kind of SEO tools . Here are some of them you can go through it. http://www.smashingmagazine.com/2006/09/22/complete-list-of-best-seo-tools/
I am using WEB CEO..It helps you to find the most effective keywords by comparing keyword popularity & competition, manages your PPC campaign with service providers, like Google Adwords and helps in validating HTML coding. Web CEO comes up with a very useful website auditing tool that can easily improve the performance of your web pages. It provides information about broken links, missing images, erroneous displayed graphics, slow pages and missing META information.
There are like lot many different requirement in SEO, So the tools are. Like in terms of judging traffic google analytics is best.
SEO tools which are best for ever are Google Adwords Keyword tool, Google Analytics and Google Webmaster tool.