I am planning a honeymoon for the middle of September. I am looking for a private luxury resort for approx. 7-8 nights. I am interested in Las Ventanas in Los Cabos, Le Meridian in Cancun, and the Four Seasons-Big Island in Hawaii. Where would you go? Can you suggest other resort choices for these locales?

I've stayed at 2 of the three. I haven't been to Cancun in 14 years, nor will I ever go there again. Trashy, crowded, raucous and ruination of what was once one of the best beach getaways in North America. (I guess you can surmise I won't be recommending Le Meridien....unless you plan to never depart the resort premises).

Las Ventanas is superb.
It was recently rated the top resort in North America by one of the travel mags, and similar awards many other times. Rooms are large and nicely decorated. Restraurant excellent. Beach out front is top notch and virtually unpopulated when we were last there, stretching for miles in both directions. Weather is desert dry, hot and predictable (which we love) and September will be high daily of 87-91. 2.7 inches of rain in Sept (rainiest month of the year there).
Great deep sea fishing and water activities. Las Ventanas has its own yacht. Very quiet, elegant atmosphere. Slooowwww pace of life due to the heat. The resort itself is luxurious (in a Mexican style of luxury) and operated by Rosewood Resorts which owns several world class hotels.
Downsides: Las Ventanas is several miles from the cities of SJ del Cabo and San Lucas; the two cities are lacking in nice places to eat, shop, etc; not a whole lot of cultural or geographic amenities aside from the water/beach related attributes. Personally, I'd be a little stir crazy there for 8 days. We never go for more than 4.

The Four Seasons Big Island is almost the best resort in the Western Hemisphere IMO, some would say THE best. The rooms are of unsurpassed class and elegance, especially for a tropical type resort. It's a relatively new place so very little wear and tear as yet. Service is the best in the US (a chain wide attribute). Great food on site. Very interesting side trips available elsewhere on the island. Very quiet, refined atmosphere.
Downsides: remote, long drive from attractions on the BI; must drive to get to some variety for restaurants; beach is good but not great and swimming not as good as Cancun or Cabo.

I'd recommend FS Big Island, but if it were my trip, I'd also stay on Maui or Kauai for a change of pace. I think the majority of people like to stay put for more than 5-6 days from reading opinions here, and I guess I'm unusual in that I like to change environments after a few days if that's an option. The biggest unique advantage of the FS BI is that there are so many other great resorts nearby which offer very different but equally enjoyable experiences. The Hawaiian islands offer a combination of variety and quality of travel options which is unmatched by any other tropical type destination in the world.

Cancun is one of my favorite destinations, but I might remind you that the middle of September is also the middle of hurricane season there also.

just based on weather (hurricane season) i'd go to hawaii. we were there in september and it was great because it was low season for tourists so it wasn't packed.

Predictions for Spring Summer and Fall are increased chances of hurricanes and tropical storms. If it were my Honeymoon FRENCH POLYNESIA BABY!!!

Would choose another destination and I am heading to Kauai this August. I go every year. The addiction has me. It is not too late for you check out weather predictions for where ever you want to go.

Those predictions were based on El Nino patterns Scientist in Hawaii have found

I'll 2nd the French Poly recommendation. And for a honeymoon, I don't think it could get much better than the Hotel Bora Bora or the Sofitel Motu, both on the island of Bora Bora in French Poly.

I remember seeing Tony Danza on Jay Leno right after he got married and honeymooned in Bora Bora.
Jay asked him how he liked Bora Bora and he said it drove him crazy, ought to be called Boring Boring.

If you go to French Polynesia I would recommend that you take plenty of money! You are going to need it. I found it to be extortionately expensive!!!!