John de Lancie's Q is perhaps the most beloved antagonist-turned-friend in the Star Trek franchise. He is both mischievous and dangerous, while remaining both fun and aloof.
As he is effectively a God, he has no fear of retribution or consequence for the most part, enjoying living his eternal life doing whatever he sees fit from time to time. He is responsible for what is potentially the most impactful meeting that Jean Luc Picard has ever had in his career and even goes on to have a somewhat strained relationship with Katheryn Janeway as well. His time on Deep Space Nine though was blessedly brief.
While de Lancie has said that he feels a return to the role would be unlikely in Picard, this would be a shame of huge proportions. While he has never been the saviour that Starfleet might need him to be, Q is a watcher who is able to offer scathing commentary on the actions and judgements of the characters we have come to know so well.
Through him, we can see their portrayal in an unflattering light, no matter how much we love them all. With a surprisingly small number of appearances across the franchise, Q has had some of the deepest impacts of any guest star through the years, beginning all the way back with his first appearance in the Next Generation's pilot. However, while that was not his finest hour, it was by no means the lowest ranked appearance. That honour goes to...