How can you choose
I love all of Sierra's adventure game series' as adventure is my favourite gaming genre. I have probably spent half of my life playing all the games from those series', the only ones I haven't played are LSL 7 and KQ 7+8.
Oh, and to answer your question, my very fav series is prolly Space Quest. The theme song, the humour, the numerous parodies of movies, the style, the puzzles and the overall fun factor all add up for a fantastic series of games.