Use meta tags properly. Use the various HTML tags' attributes, like the alt tag for images. Wherever possible, follow the DOM in your page structure (ie: H1 tags are the most important headings on a page, followed by H2 tags, followed by [if any] H3 tags. Don't just rely on styling.)
Structure your content appropriately: interlink, cross-promote pages on your own site if applicable. If you have a graphic design business, or a mural business, or a are a photographer, put some descriptive text beneath some of the key images in your gallery. If you have a web design business, or some business where you put together a series of smaller tasks for a project, do up some case studies. This is always tricky: you have to find the balance between using the right amount of keywords without sounding like an infomercial.
Content is key - but the proper content is better than lots of badly-organised or poorly-displayed content.
No single thing will be "the key element", since they are all subject to abuse. Create a well-optimized page with no META tags, and a similar page with properly formatted META tags will have slightly more weight (all other aspects being equal.) On the other hand, try to spam or stuff your tags full of text that doesn't match the actual page content, and you could find yourself penalized somehow. I don't know enough about the different search engines to know if, for example, Google adjusts the PageRank *downward* for pages / sites that try to game the system; but I wouldn't be surprised to learn that they did.
