Which is best in seo purpose adding meta tags in custom field or putting into seo plugin like headspace or all in one seo.
it really doesnt matter. YOu can accomplish everything that an seo plugin does with a little template work and custom fields. Most of the plugins out there use custom fields. If you have the time and the know how I would use the custom fields option simply because it will reduce your sites load time. The only downside I see is that the homepage keywords may not be able to be worked out like an seo plugin would.
It highly depends on your need, I am not using any of the seo plugin, because I don't think that over loading deserve the plugin.