I realise 'favourite' is the wrong word but could not think of another. I say religious celebration or service because I am really interested in the answers from all faiths. The question was just triggered by the fact that my 'favourite' mass of the year is imminent.
I just love the mass (RC) on Holy Thursday which remembers the Last Supper. It is so beautiful, solemn and reflective, it usually reduces me to tears. It has a real emphasis on service, the priest washes the feet of some of the congregation and those attending wash each other's hands.
It strenthens my faith as it really brings home how Jesus made such a terrible sacrifice in such a gentle and humble way and the real communion of Christians. It also gives so much time for reflection. There is a period of 'watching' after mass before the Blessed Sacrament which just gives time to sit and think whilst in a very tangible presence of God.