It has been a pleasure playing with you.

I agree with Michael's answer. However, be aware that a native speaker (at least in America) would almost never say "It's been a pleasure" or "it was a pleasure" because it's very formal. I can imagine a Victorian Englishman saying "it's been a pleasure." But contemporary Americans would use the word "fun," and say something like "It was really fun playing with you." You could also say something like "That was a great game!" Another super subtlety is that "it was a pleasure playing with you" sounds somewhat sexual, because to "play with someone" can mean doing sexual things, and "pleasure" also can have sexual connotations. I would stick with "fun."

You can say either in this case. I would probably use "was" if the game had finished, and "has been" if the session had finished but we planned to resume the game at another time.