While security is the degree of protection against danger, damage, loss, and crime, being truly anonymous is a matter of view and perception. As Chao very well pointed out, things like behavioral analysis can be used to narrow down on an individual, but are insufficient to convict him.
Not identified by name; of unknown name.
If we use that definition then just having behavior analysis experts cannot identify you as a particular individual. Only narrow the search area.
Therefore, the question to be asked is not "how to be completely anonymous online" but instead "How do hide from someone online". The reason is that different people use different tactics to find individuals.
Allow me to explain...
VPNs, proxies and Tor anonimise you to an extend that a particular 3rd party organization cannot later "backtrack" you. However, they do reveal your identity to the first node in case of Tor or to the proxy server itself.
If the node/servers keeps logs, you're identified then and there. If not, the telecom authority could hypothetically still have connection logs between your IP and the server, which could then get you identified.
What I'm trying to explain is your question is too vague in order to give a definitive answer. Just as security, anonymity is a degree of perception towards being anonymous.
To be frank, no one can be truly anonimous in real life or virtuality, since for any kind of communication an identity is required, and although we can give this identity any name we want, it is the core of our persona that cannot be faked, and therefore we cannot be anonymous.
lacking individuality, unique character, or distinction: an endless
row of drab, anonymous houses.
If we apply the same concept in technical terms of TCP/IP, then without a unique IP we cannot communicate, and having anything unique is against the second definition of being anonymous.