Twitter is definitely valuable.
After that, "best" depends on your goals. Meetup is great if you want to create local groups. Flickr is great if you want to share photos, and YouTube and Vimeo are great if you want to share videos. And, of course, there are tools like Match.com and eHarmony for dating. So I suggest defining your own personal or business goals, then looking for a best site that matches that purpose.
In terms of navigation, it is again more a matter of personal taste and perseverance than of ease of navigation. Some seem natural or "intuitive" to some people, and others to other people. One issue there is generational differences: I think LinkedIn thinks more like a baby-boomer, and Facebook more like a Gen X man or woman. There's also a link between purpose and comfort. My wife writes haiku and loves twitter. I write articles and stories, and love HubPages.
All of these tools will be difficult at times, and not do what we want. They will also fail, as the Internet is inherently unreliable, and for other reasons. In my view, living with this unreliable and uncomfortable technology, and persevering to business or personal success means succeeding through discomfort, not finding a perfect tool