the majority of them work, it all depends on how much you put in. ie input ---> output science energy stuff. There was no spell on this site posted without the knowledge that it worked for the caster one way or another. they didnt post it because it didnt work, they posted it because it worked. you should look at wishing spells, or summoning and protection spells... they work pretty well. However, the best spells out there (for you) are the ones you make yourself.
to quote a song from a movie i liked as a kid 'practice practice practice'
though i am not about to likes ALL the sites spells that work or not, you can figure it out yourself simply. when you read a spell ask yourself does this sound like a reasonable spell? [i've seen some spells that example say break a pencil in two and that's it, you could try adding onto it, like a chant or visualization.]