Best Hilton property in Bangkok

I have read the reviews, and see they have added another property. We have six nights booked in koh samui Conrad. I need to book two nights prior, and two nights after the Samui leg. I was trying to go somewhere else but flights are too long, and expensive for the short trip.

I will be with my wife, and this will be the first time we will be in Thailand. I hate to waste the free credit card nights on a night that only cost $100-150, but not sure if I will be on another trip to use them. I will have three nights covered via the card, and will pay the fourth on points earned to attain diamond.

Out of these three properties what would the experts recommend?

Millennium Hilton Bangkok hotel
Conrad Bangkok
Hilton Sukhumvit Bangkok

I am currently gold, and will probably spend the extra 30k to get to diamond for this trip. So lounge access is a good thing, but location to see everything is also important. Looking for the best overall location. (room, lounge, staff, and location)

thanks for any help as this trip has already been a lot of work in planning.