Where did you find the thistle hotels at 100 per night?

Not sure about the US but the local discounters here in London are always clearing out Thistle rooms.
Cheapest is probably lastminute.com, they don't actually says Thistle x - they say just say something like 4 star hotel Victoria, when you read the description of the hotel concerned its soom becomes clear what Thistle it is.
They had Thistle Victoria for 59 GBP which I guess is around $100.
I know Active Hotels has all of them available for 69 GBP until end of January which is just over $100.
Personally I'd use the Thistle Auction Site, the London hotels often go for well under 50 GBP per night which is probably what they're really worth.

Regards the Thistle discussion I agree with everyone.

Yes, it is questionable for Priceline to treat them as genuine 4 star hotels.

Yes, if you're paying full price, i.e. > $150 for a hotel, steer well clear of Thistle.

But, hey at $50-$75 a night it must be a bargain to stay at a Thistle.
After all I'll fly Aeroflot or Air India to London if the price is low enough.

The 4* Mayfair-Soho hotels on Priceline seem to have the best reviews and to be the "nicest" of all the zones. So if you are worried that a hotel like the Thistle Marble Arch is not up to your standards (at a price that comes out to about 40 pounds with taxes and fees all in for a double room), but you are otherwise okay with the Priceline concept, be sure to restrict your bidding to the Mayfair zone.

I've had three different Holiday Inns in London through Priceline (both 3* and 4*). I can't say the hotel experiences were transcendental, but they were well above adequate, and twice I've been upgraded to executive rooms. If you want a gorgeous boutique hotel, of course Priceline simply will not be right for you.

Would suggest you consider not going to a chain hotel however many stars it has.

My preferred London hotel is the Darlington Hyde Park. I don't think its bothered to get any stars, but the service, cleanliness and hospitality is second to none. It hasn't got 4 bars, and acres of marble but just do some basic research on the net to see I'm not the only one, try it once and you won't go anywhere else in London. Rooms this time of year with an excellent breakfast are reasonable at about $120.
Its costing you a lot of dollars to get to London, so for just a few dollars more than the Priceline lottery you're assured of a memorable stay.

To answer the original question. Don't know Hilton Kensington apart from going past it on the outside.

Of the others the Rubens at the Palace seems to stand out head and tails to me for both position and quality. If this is your short list I'd have no hesitation in picking the Rubens opposite Buckingham Palace from that list.

Off the list I'd also have no hesitation in also recommending the Darlington Hyde Park also mentioned by a couple of people here.

The Rubens is in a better position and will impress the folks back home more. The Darlington Hyde Park is the wise buy, a lot of the customers are like the person on the previous post - tried a lot of hotels in London and have decided on the Darlington as their London base.

Pick either one of those and I'm sure you're assured of a great stay in London.

We've stayed at the Hilton Kensington. It's not too far from the Holland Park underground station as I recall, so the location is fine. The rooms were clean, but very small; if you're expecting a regular Hilton, like one you'd find in the U.S., you'll be disappointed. Also, the a/c during the summer was borderline.

If I were in your position, I might try the Reubens. It looks great fromt the outside, it's in a good location near Buckingham Palace & Victoria Station, and it gets good reviews in this forum, as well as on trip advisor.

It looks like in your proce range you amy also be able to afford the Kensington Close Hotel which is in a great location in Kensington off High Street. The romms are clean and cozy, but not very big. Sometimes you can land a room for 75 pounds if you book at their websight by clicking on "SPECIALS"

http://www.kensingtonclosehotel.com/

As far as your other choices...

-The Hilton is a bit out of the way

-Rydges is an Australian chain that has a good reputation. The location is so-so though.