Spring for sure - especially April and May. Everyone forgets about skiing so the hill gets really quiet and it feels like you and your friends have the place to yourselves. Stability improves and coverage is at its best so you can start getting on to bigger lines and finally hit objectives you've waited all year for. You get sleeper pow days so resort skiiing starts to feel like cat skiing. Even if you haven't skied park all year as soon as you have a few slushy days you get the craving for it again and start wishing you'd hit it more. You remember skiing slushy bumps is one of the most fun things ever, dodging and jumping over rocks and dirt and streams. Cooling down after a hot day's skiing with cold beer and an evening full of sunshine.
Most of all you start getting that sad but happy nostalgic feeling that it's all coming to an end - making sure you make the most out of every single day you have left whilst reflecting on what a good year you and your friends have had and how blessed you are to be able to look forward to another season of it next year.