Hey, I sort of had the same thing. I find it hard to know whether my childhood memories are true or an imagination of what someone else has told me. For example; on the beach with my mum in Egypt at night and the sand turning green where our footprints had been.
I'm also trying not to answer with 'lovely holidays' because that seems like a cop out and not very specific. The luge in la clusaz. Dad's jeep and mixtapes. Ping pong playing. Spending hours in the lake pretending to be a mermaid. I don't know, I genuinely couldn't answer this question. I know I must have had a lovely childhood. Thinking about it though, I spent a lot of it alone. Playing Sims or Spyro the Dragon. Putting up a side of the ping pong table and playing for hours, my right hand against my left hand. Pretending to be a dog and eating crisps from a bowl (!). But one memory, my 'best' memory, I can't answer that.
I'm glad you managed to think of a few good memories, despite of struggling with this one. I find it's a lot easier to go to 'the bad things.' I can list those off very easily, so I guess it's nice to refocus and hopefully these will come easier next time.