Of those listed, my favourite has to be Mew. The original, Uber for four generations, and still one of the most versatile Pokemon around.
Considering one-on-one battles, however, Jirachi and Victini are gonna be in there I reckon, the former resisting all the Psychic STABs and having flinchhax while the latter hits hard with V-Create crushing Celebi and Jirachi in particular. Azelf and Uxie also have more targeted stats in offense and defense respectively, compared to the flat 100s of most of the cute legendaries. Still, Mew has movepool like crazy; obvious sets would be Nasty Plot with Shadow Ball (for the psychics) and Flamethrower (for Jirachi), or alternatively physical with Swords Dance, X-Scissor, and Earthquake.