Q:

Who is the legal owner of the Twitter account?

I often see someone on Twitter saying something like: My account = my rules. And I always wonder…is this actually true?
Who is the legal owner of the Twitter account?
Is it Twitter or the user who created it? 
And who is an owner of the posts?

A:

Twitter, as the owner of the site, can make rules for how users will conduct themselves. It has delegated extensive authority to account holders to moderate comments posted to their accounts, including the ability to delete comments for any reason or none. This effectively allows the account holder to make rules, within the very broad limits of the Twitter TOS. 
A poster holds the copyright on any post that s/he has created. No one may copy it without permission, just as for any other copyrighted work. (Posting grants limited permission to copy, under the site's TOS, as I understand it.) Similarly, no one may create a derivative work, such as an edited version, without the author's permission. 
But a poster has no right to post, nor to have posts remain displayed. The account holder, or twitter itself, may delete posts at any time, and may also block a poster, for any reason or none. The poster can post about this on his or her own twitter account, or elsewhere, but may not insist on being displayed on someone else's account.
Note also that the copyright could only be enforced by suing the alleged infringer. Since most Twitter posts have no commercial value, economic damages would be hard to prove, and such suits are rare and do not usually go anywhere. Judges tend to be hostile to copyright suits over matters of principle when no economic harm has been done.

