Traditional 9, with emphasis placed on Law/Chaos/Evil/Good being actual forces of power.
Personally, I feel it makes games a lot more interesting when you know that an increase of evil wizards and priests means an actual increase of Evil. And like KM said, there's something awesome about actually USING them as forces, with waves of evil, or evil outsiders physically reeling from not an aura of good, but an aura of Good.
It also helps give a bigger emphasis on redemption. I found the 4e "alignment doesn't matter, all monsters are evil" to be horribly boring from a story and character stand point - I always liked it when players spare enemies or try to convert them back to good, and having the nine elements and the full "Alignments are powers" helps that. The rogue gets a new quazit buddy and shows him to do a few tricks that doesn't hurt people, but still gets a good laugh, and how to have a grand ol' time just tavern hopping and being sporadic while still helping people out. The paladin shows a devil what the law can be when strengthened with good and honor. The cleric uses a spell powered literally by Good and helps a fallen enemy see the error of their ways by filling him with the glories of what he once was and what he once served.
Lastly, nine alignments, in my opinion, lead to more interest and sketchy alliances. Chaotic good, chaotic neutral, and maybe some evil characters team up to bring down a tyrannical dictator. The lawful evil warlord helps a noble leader bring down a group of saboteurs and rebels out of duty to Order.
I can see "We're unaligned, they're evil, let's go down to murder town" being fun for a chips and soda game, but...well, I already have Risus for that ;p