Looking on the internet deeply has found these results:

It is my pleasure to assist you. is the most popular phrase on the web.

It was my pleasure assisting you. 113,500 results on the web	It is my pleasure to assist you. 166,800 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It was my pleasure assisting you. It was my pleasure to serve you. It was my pleasure, Sarah. It was my pleasure, really. It was my pleasure, Eli. It was my pleasure, sun. It was my pleasure, and yours. It was my pleasure, really.	It is my pleasure to feed you and prove the bounty of our Earth. It is my pleasure to welcome you to Afghan air space. It is my pleasure to you show progress. It is my pleasure to welcome you all to Camelot. Your Majesty, it is my pleasure to present Madame de Barra. It is my pleasure to award you this honorary diploma. It is my pleasure to deliver your signing bonus personally. And now it is my pleasure to introduce a young man who's represented our school at model U.N.

It was my pleasure assisting you.

113,500 results on the web

It is my pleasure to assist you.

166,800 results on the web

More popular!

It is my pleasure to feed you and prove the bounty of our Earth.
It is my pleasure to welcome you to Afghan air space.
It is my pleasure to you show progress.
It is my pleasure to welcome you all to Camelot.
Your Majesty, it is my pleasure to present Madame de Barra.
It is my pleasure to award you this honorary diploma.
It is my pleasure to deliver your signing bonus personally.
And now it is my pleasure to introduce a young man who's represented our school at model U.N.