Port Louis, Rodney Bay, or Marin in
Martinique are all good choices, if you want
marinas with lots of services, restaurants etc.....but I would pick up a
mooring ball in the SMMA around the Pitons/Soufriere in St. Lucia. Very friendly people - look up the
head ranger there - Peter Butcher - who'd one of the friendliest officials I've run across anywhere. And make friends w/ one of the locals who can provide all the water or land taxi and guide services you might need (Peter can recommend/arrange, as well). It's a bit closer to the airport than Rodney Bay, and a lot more beautiful (few spots on earth more so)....and has great
diving, snorkeling, hiking...and restaurants ashore nearby.
For European flights, though, Martinique may be the best....and, it's a very safe and clean island, with plenty of great
food and wine.....not to mention mountains, waterfalls, beaches,
rum distilleries.....and French culture (love it or hate it!)....
Another alternative would be Union Island in the central Grenadines. No international air
service there, but easy puddle-jumpers from there to
Barbados. And plenty to do there and with trips to the
Tobago Cays, Mayreau and other nearby islands.
So many great choices.....so little time!