There really is no best item. Since the Grand Exchange updates so much. prices will change often. So one day you could be making 50gp profit with an item then the next day the item could lose you 50gp.
Voodin is right, prices are going up and down like crazy. Just alch something that has loss of gp, but not a lot. Honestly, if you have rich, alch needles lmfao.