What do you think is the best strategy to get a hit single? Releasing a single that suits the particular season its being released? Eg releasing an upbeat anthem in summer or ballad in winter or going with the strongest songs in order?

Be a big star. Make a catchy ass song. Hype it up. Make a good video for it.

remixes, discounts, payola

Really good song, be famous and if you are losing relevance or have not released anything for a long time sure it's a theme song or whatever to a popular movie or something. Example: 桜流し

Strongest songs obviously. Roar is seasonless.

It isn't the strongest song either tho?

Strongest
Release a messy, not catchy upbeat song will still flop in the summer

A wacky single - Jan to March
A upbeat, summer single - May to July
A ballad - September to Dec

I don't think it matters if the song is catchy. Releasing a ballad during fall/winter and an upbeat during spring/summer is not always true if the song is catchy and there are hundreds of examples for that.