It's no secret that Sanji is probably the best cook in One Piece world, who knows maybe in reality as well :P. That could be one of the reasons why he is Luffy's favorite. Sanji's main dream is to find the "All Blue" where it is said that all kind of fishes are in that ocean. Actually it's just a legend and nobody can really prove it's existence, but I'm sure that Sanji will find a way to find this place. His other hobby is probably even more important for him. I'm talking about running after hot hentai woman all day long. No for real, he could do this all day long.
The only problem is that during all these years of watching One Piece Anime or Manga I have never saw him score some chick. I think this is mean by Oda, but since he is the author it is his decision after all. Although Sanji loves all pretty females on this planet his most beloved are Nami and Robin. This gentleman would rather risk his own life than to allow somebody to hurt these two girls. Sanji's biggest rival in the Strawhat Crew is Zoro, I don't know why but these two don't really like each other and are always fighting and insulting themselves. Maybe it has something to do with the fact that Zoro and Sanji are being next to Luffy the strongest in the crew and this may trigger some kind of rivalry.
So lets get back to Sanji's cooking skills. What can he cook? Well he can make a meal out of any ingredients that you bring him and to top it all our master chief will make you the most delicious dinner ever from anything. Yes, he is a true manga genius.