My vote is for Plaza on the River (a Park Plaza hotel). Just got back from a 3 night stay as CC Gold. Originally scheduled for 2 nights, but sickness forced us to stay another.
The first night was booked with 50,000 points. I emailed and asked for an early check-in and riverview upgrade. The response was yes if there was one available. Upon arrival, I was upgraded to a fantastic one-bedroom suite with a view of the Thames. The standard award is a studio room with no view. I didn't do the nor-1 upgrade, which offered an upgrade to a suite with a balcony view of Big Ben and Parliament for around 30 pounds. Definitely thought about it though. The second night, I did a points and cash room, so I could take advantage of the latest Park Plaza promotion for a bonus 50,000 points. So I used 15,000 points and $190 to get another 50,000 points. They kept me in the same room. Unfortunately, for the third night, we did get switched to a non-view room, but still a one-BR suite. At least my wife was able to book with her account and also got the PP promotion.
The Plaza on the River is in the same building as the Park Plaza Riverbank. I thought about doing the second night points & cash in the Park Plaza Riverbank for the promo, since it would have cost less, but I loved the Plaza on the River suite. It was huge and had a kitchenette, fridge, washer/dryer, and Nespresso machine. Bathroom was also very large with Gilchrist & Soames toiletries. Crib for our 10-month son was brought up promptly. Very modern hotel with incredible customer service. Vauxhall is the closest Tube stop to the south, and it's really less than a 10-minute walk. We walked across Lambeth bridge to get to Westminster pretty easily.
I never got the CC Gold welcome amenity, but besides that (and being ridiculously sick), I really enjoyed the stay.