score:3

Accepted answer

If I understand right you have a many-to-many relationship between articles and tags but the navigation property on the tag side is not exposed (there is no ICollection<Article> in Tag). I think in this case you have to start from the articles to get all used tags along with the information how often they are used in the articles:

var tagQuery = db.Articles
    .SelectMany(a => a.Tags)          // flat enumeration of all used tags
    .GroupBy(t => t, (k, g) => new    // k = key = Tag, g = group of same tags
    {
        Tag = k,                      // the tag
        Count = g.Count()             // how often does it occur in the articles
    })
    .OrderByDescending(g => g.Count); // most used tags first

If you want all tags sorted descending, call 

var tagList = tagQuery.ToList();

If you just want the tag which is most often used in the articles, call

var mostUsedTag = tagQuery.FirstOrDefault();

In both cases the result is a collection/single object of an anonymous type which has the Tag and Count as members. If you just want the tag(s) and are not interested in the Count anymore you can project before apply ToList or FirstOrDefault: tagQuery.Select(anonymous => anonymous.Tag).....

Edit

Just saw the Edit in your question. Now, when you have an Article collection in the Tag entity it is easier:

var tagQuery = db.Tags
    .Select(t => new
    {
        Tag = t,                      // the Tag
        Count = t.Articles.Count()    // just count the number of artices
                                      // the tag is used in
    })
    .OrderByDescending(x => x.Count); // most used tags first

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