Depends on you preferences - is it price, location or hotel facilities?

HI Silom has a very strict policy: As platinum member you will be upgraded to a Premier room (which are in the refurbished tower and much nicer and larger than standard rooms). Apart from some fruit they will give you nothing else (e.g. internet, breakfast) and especially no free access to the lounge (which you may buy at ~800 B, lounge is really nice).
However, prices at this property are very low and the Premier Rooms are really nice (and large!). Location is not exactly central, but the BTS is quite close (and of course taxis are there as well...).
--> This property (imho) is the best choice in terms of value!

HI Ploenchit can be a hit or miss depending on their occupancy: You may be upgraded to the Executive floor (incl. a not-that-nice Lounge) but you may also end up in a standard room on a higher floor. No free internet or breakfast.

HIX Siam is a low-budget property and therefore does not offer any better rooms and limited service. However, you will have free internet and basic breakfast (standard at HIX).

CP is a modern and nice property. As Platinum you will most likely be upgraded to a deluxe room, could be a small suite if you are very lucky or a frequent guest. I personally do not like the location that much. Prices are sometimes rather high as well. If the difference to the IC is < 1000 B I would invest this money and get a room there

IC definitely is the best hotel of all these, but they hardly honor Platinum benefits. Maybe some slight upgrade to a higher floor. Depending on the lenght of your stay it may be worth investing in Ambassador status. Club lounge at this IC is great, especially with wonderful views over the city´- so if it's a short stay you might think about booking a club room which may also lead to an upgrade to a small suite if you are lucky (guaranteed if you have Ambassador status).


Hope this helps