More formal:
* A wonderful evening
* A delightful evening
* A memorable evening
* A pleasant evening
* A successful evening
* A most enjoyable evening
More informal:
* An awesome evening
* A fantastic evening
* A killer evening
* A blast
* A real treat
* A night to remember
* A great time
Specific to the event:
* A fun-filled evening (if it was full of activities)
* A relaxing evening (if it was calm and peaceful)
* A delicious evening (if the food was great)
* A romantic evening (if it was a date night)
You could also use a phrase that describes the feeling you had:
* I had a really good time.
* I enjoyed myself immensely.
* It was a night I'll never forget.
* I felt so happy and relaxed.
Ultimately, the best way to say "a great evening" depends on the specific situation and your personal style.