Picking a favorite beach is a cool thing - it is like picking a favorite luxury automobile, it is hard to make a bad choice. My Rolls Royce of beaches is Waimanalo Beach in Hawaii.
Waimanalo Beach is located on the eastern side of Oahu and it is easily missed because of the other fine beaches and tourist attractions on the island. Specifically, Waimanalo Beach is just south of Bellows Air Force Station (which I really don't think is in use, because I never see planes coming or going) starting at Waimanalo Bay State Recreation Area and going down to the Waimanalo Beach Park - at this point the Kalanianole Highway (try to say that one fast) starts running right on the Pacific Ocean. From a broad perspective, Waimanalo Beach is between the Kailua/Lanikai area and the southeastern point of Oahu.
Don't worry if you have never heard of Waimanalo Beach - we only found it by accident; we got a bit lost and being in Hawaii, we found a beach to re-group at!
Like almost all Oahu beaches, Waimanalo has a beach park which contains a picnic gazebo, kids play area on grass, restrooms and outdoor showers - the Kailua and Waikiki beach parks are a bit neater, but the Waimanalo one gets the job done.
In terms of the beach and water, the sand at Waimanalo is very deep and a nice golden color - it is a larger grain of sand, so it brushes and rinses off easily. Since this is a rather unpopulated area, the palm trees come right up to the sand and create many shady spots to escape the sun. Waimanalo Beach itself is not overly wide and tends to be steeper than Kailua or Lanikai - from this perspective it is what I picture as a tropical beach in my mind.
The water is an amazing color and probably one of the reasons this beach is my favorite; somehow the water is a silvery-sky blue tone that I have not seen anywhere else.
The off-shore reefs in Waimanlo Bay are not as significant as at Kailua Bay, so the waves tend to be a bit larger than Kailua or other east Oahu beaches. This is nice on minimal swell days, as there will be enough size to the waves to at least get some fun body surfing in - on the larger days, if you are not an experienced body or Boogie board surfer you may want to stay out the water at times. Since it is a sandy bottom, if you get rolled, you should just come up covered with golden sand, not cuts or bruises.
I have yet to see Waimanalo Beach crowded, even on weekends during the Holidays. If you want to get away from anyone, literally anyone, just walk a hundred yards away from the beach park and you will be alone! This is the place to go for beach solitude in Hawaii - another reason for my #1 ranking.
This part of Oahu is not very populated and in fact, can be a bit rough - it is not as polished as Kailua or Hawaii Kai. This is a Locals area - local Hawaiian folks who have not bought into the whole tourism trade and if you grew up in Waimanalo, you would be one tough dude! There are no hotels and very limited vacation rentals in the small grouping of houses that is Waimanalo Beach. If you want something to eat, you need to drive north a few miles to Waimanalo where there is a L and L's Drive In (Local food which is AWESOME) and couple of fast food places and small grocery stores - this is definitely not Waikiki! Again, another reason that this is my #1 Beach - it is old school Hawaii.
Make sure that you plan on arriving early to Waimanalo Beach, because the Koolau Mountains on the Oahu east side seem so close you can touch them, which makes for an early shadow coming across the beach. In a sense this is nice, because the late mornings can be very hot, but since the afternoon shade comes quickly, you can enjoy the waves without getting a nasty sunburn!
Even though Waimanalo Beach is mainly a drive to beach, the water is unreal, there are good waves to enjoy, the beach is amazing and it is uncrowded - what else could you want in a beach?
