different spells work better for different people, and as mentioned you should write your own spells or alter spells to work for you.
on this site, flying, illusion, invisibility, levitation, teleportation, immortality, time travel, dragon, faerie, mermaid, vampire, and werewolf spells generally do not work. they are fictional, with the possible exception to calling upon faeries or such for energy in spells similar to calling on a deity. a spell to change into these creatures definitely do not work. grey area's are weather, mind reading, and death spells. there might be more but off the top of my head i can't think of any.
magick is energy found in nature used in spells to create changes in our daily life. this energy is a part of nature, so it has to work within the confines of nature, this is why spells that create physical change and improbabilities don't work, while spells to bring luck or love do. spells work with forces and energy.