Hate to get all philosophical and stuff but it's hard to pinpoint a favourite day ever because I feel like life is full of highs and lows that you can't really compare. You've got loads of different kinds of happy and it's really damn hard to choose a favourite day, maybe because I've forgotten many of the details in the past or whatever.
That being said though, in recent times today was pretty good (Spurs 3-2 Ajax #COYS), so was the day I got my maths results from this sem (probably the best results I've ever gotten for any subject), the exhilaration at the end of exhausting hikes in Year 9 and 10, winning an academic decathlon with my mates in Year 9, finishing the tennis season a few months ago, and loads more I can't even name. Probably the thing that all those moments and days involve is pain leading up to them and effort (watching Spurs 3-0 down at half time on aggregate, all the revision, energy loss and hard hiking the whole time (literally crumpling on the train on the way back after Year 9), the struggle of even completing some of the tasks, and the toughness of the training sessions), then finishing in pure happiness that makes the pain worth it. That's probably the key point when it comes to having a good day for me, even though I can't pick any one day