Favorite is, without question, Pine Valley. I have dreamed of playing it since I was a caddie and would hear the St. David's members talk about occasional trips there. So many amazing looking holes. Disclaimer: I am a bit nervous if this happens about bringing my A game. (note: earlier this year the answer might have been NGLA, but now I've played it - and it exceeded my very high expectations!).
Least favorite - I am going to take this question to mean least favorite highly ranked course. Using Golf top 100 world, that has to be the current #50 - Trump International Golf Links Scotland. While I think the Donald did a great job at Doral, and I know Ran sings its praises, the look of the course just doesn't appeal to me. It seems over-bunkered and the mowing lines look jarring. Perhaps this is just how it comes across in photos. I am not opposed to giving the course a try, but if/when I go to Scotland, courses steeped in history will be a lot higher on my list of priorities than TIGL.