Well it seems like its been a while for me, but I'm a 19 year old instructor and it hasnt been that long since I was just a 13 year old regular diver. I've been to all those listed places, and I would have to narrow it down to St. Lucia or Antigua, but definitely leaning towards St. Lucia. There are many sites on the west (lee side) of the island which means usual calm conditions and easy diving. Lots of marine life, an amazing dive at Anse Chasenet(sp?) and I remember a fun wreck, not too deep that was sunk a few years before I dove it, by now it should have decent growth on it.
Antigua was nice, but all the good sites were quite a ways off the west side of the island, almost half way to Nevis. It's been about six or seven years since I've been there, but they were enforcing their marine park pretty well and from what I've heard theres still some great diving there. Good luck and safe diving!