What is Nirvana? What is the practice leading to ultimate happiness in the light of one's own experience? What is the path or practice leading to ultimate happiness in the light of one's own practice which has lead you to achieving true happiness?
I will jump into the fire of people's objections to posit that there is no achieving of true happiness. My intention is to reveal the truth that does exist beyond the bounds of our human expectation of ending suffering and attaining some new state or place where happiness is forever.
Zen does not look to attain or achieve a state but to awake to what is here. The Buddha replied when asked who he was: Awake. Any path can lead to awakening. The Buddha clearly defined the Middle Way, but it is not the only way. Even Buddhists who followed false teachers found awakening.
The intention is the key. If we intend to promote ourselves and our elusive and illusive personal self, we are bound to fail.
So to relate this to the Buddha's message here are the principles of the eight-fold path. Right intent or intention is here among them. 
http://www.buddha101.com/p_path.htm


  The eightfold path is Right Understanding, Right Intent, Right Speech,
  Right Action, Right Livelihood, Right Effort, Right Mindfulness, and
  Right Concentration.


Note that right does not mean right as opposed to wrong. Right can mean when things go in the flow or without resistance (same page further down)


  The meaning of Right has several aspects, and includes an ethical, and
  a balanced, or middle way. When things go "right", we often experience
  a special feeling inside which confirms that this is the correct
  decision or action.


Further down the same page about intent


  Right Intent:
  
  The second step on the Eightfold Path is Right Intent. This is the
  step where we become committed to the path. Right Understanding shows
  us what life really is and what life’s problems are composed of, Right
  Intent urges us to decide what our heart wants.
  
  Right Intent must come from the heart and involves recognising the
  equality of all life and compassion for all that life, beginning with
  yourself.
  
  Right Intent means persistence and a passion for the journey. Setting
  out to climb a high mountain means you must understand the lay of the
  land and the pitfalls, the other team members, and the equipment you
  need. This is similar to Right Understanding. But you will only climb
  the mountain if you really want to and have a passion for the climb.
  This is Right Intent. The mountain we climb here is our journey though
  life.
  
  To summarise, Right Understanding will eliminate ignorance. With Right
  Intent and correct understanding, we then remove desire, which in turn
  causes the suffering defined in the Four Noble Truths.


I rest here and leave it to those interested to find any other quotes of interest on that page. Thank you for the question.