I do not believe in favourite keys.
Every song is different. There are reasons why certain songs are played or written in certain keys. It is easier to play or sing seems to be the most reasonable explanation. Certain chords do sound better or richer if played in another key might also be a factor to be taken into consideration. Horns are better suited to flat keys as most are scaled in E flat. Male female is also a factor as very few males can sing a female song in the original key and vice versa. Bagpipes are another ballgame altogether as the one key drone is limiting. So all in all it is important to play in a key taking everything into consideration. Sometimes a compromise is needed.