楽しい時間を過ごせたことが（一番）良かった。makes also sense to a question, 何が（一番）良かったですか？

Thanks for your reply, that makes sense! Thank you!

"楽しい時間を過ごせてよかった。"or "楽しい時間を過ごすことができてよかった。" is correct. "過ごせてよかった" is more natural in daily conversation. "過ごせて" means " i could spend " , and "過ごすことができて" means " i could spend" too. So you don't need to use both.