For me
Best: British Airways Airbus A320-200 from London Gatwick (LGW), UK to Naples (NAP), Italy, in December 2018. Very friendly cabin crew / flight attendants, and they would happily talk to me in my making of a trip report video (which I will eventually post onto YouTube). I also always enjoy flying with EasyJet, and on my first flight during the Covid-19 pandemic (and indeed my first holiday abroad during the pandemic!) from London Gatwick (LGW), UK to Malaga (AGP), Spain (in June 2021), the cabin crew were very friendly. When I said this was my first flight during the pandemic, the cabin crew replied something along the lines: "Welcome back onboard! We have done a lot of measures to make you comfortable and not scared" and I replied: "and in fact, I'm not scared at all. I'm super excited to finally fly!". And another great thing: my first pandemic flight was not just on any aircraft, but a brand new Airbus A320neo!
Worst: Alitalia Airbus A319 from Rome-Fiumicino (FCO), Italy to London Heathrow (LHR), UK in July 2011. I was desperate for a water bottle, so I called the cabin crew / flight attendant asking for one. She initially refused. My mum then complained to another cabin crew member, and after so long, I finally received the water bottle. It was so hot that day that I was desperately thirsty. Thankfully, when I flew with Alitalia again in April 2015 from Rome-Fiumicino (FCO) to Milan-Malpensa (MXP) (both in Italy) on an Embraer 175, the cabin crew were much friendlier and welcoming.