makrom

A "small" comparison to the other accor properties you mentioned (visited Banyan Tree 1BR suite, So Sofitel wood room towards Sathorn Rd and Water towards Lumpini, Sofitel high floor room)

Style:
1) So Sofitel: Lobby, restaurants, hallways, rooms, it simply has a very nice luxurious modern flair. My personal favorite.
2) Banyan Tree: Luxurious modern Thai style with nice touches all around, definitely a 5* hotel in that regard. It's very much about personal preference, I would rank it 2nd.
3) Sofitel: Not bad either, though I prefer the So Sofitel's more extravagant approach to the Sofitel's rather classic elegance.

Views and location:
1) So Sofitel: Really love the views over Lumpini Park, I even consider them strong contestants for Bangkok's #1 . Sathorn Rd side is ok but less spectacular for sure. I like the location as being both close to Lumpini Park as well as having Silom Rd in walking distance. MRT is quite close and BTS within walking distance too, but xing Sathorn Rd is not the most fun activity. Wedged between Rama IV Rd and Sathorn Rd, it is a high traffic area and as such I would like slightly more sound proofed windows (it's not exactly bad but definitely audible).
2) Banyan Tree: Though quite close to the So Sofitel, feels like a more remote area. MRT and Lumpini Park still within walking distance, but a bit further than I would like. Good area of town, next to the Sukothai Hotel and the German embassy, but not much to do around there. I like that it is a bit offset from Sathorn Rd as it helps giving it a more tranquil atmosphere. Views strongly depend room location. From rather boring over acceptable to outright stunning, best case is probably high floor towards north-west.
3) Sofitel: Location wouldn't be my first choice as I am not a big fan of Sukhumvit Rd. With the right rooms you can get some nice views of the Sukhumvit skyscrapers or even above them.
So Sofitel has best average views, Banyan Tree might have the best possible view. Sofitel isn't bad either but I prefer views over Lumpini Park.

Room size:
1) Banyan Tree: Comparing a suite to regular rooms is borderline nonsensical. From what I know even the smaller rooms are good size, but who knows... For a couple of 2, the 1BR suites have a very good size and the room separation is nicely done.
2) Sofitel: Nicely spaced rooms, though not spectacularly roomy.
3) So Sofitel: This is probably where I would rank it last. Rooms are not cramped by a long shot, but you won't enter it and get a wow effect. They have some nice designs that help making more efficient use of space but there are other aspects where it's noticeable that it is simply less roomy.

Tranquility:
1) Banyan Tree
2) Sofitel
3) So Sofitel, though it probably doesn't aim at that anyway.

Breakfast:
1) Sofitel very slightly in front of
2) So Sofitel again slightly in front of
3) Banyan Tree (Club lounge).
All 3 work on a good 5* Bangkok level. In contrast accor's 5* MGallery properties Muse and Vie are clearly worse in that area (not outright bad, but definitely on a lower level).

Culinary in general:
Hard to name an overall winner, though I am slightly leaning towards the
1) Sofitel.
2) Banyan Tree: As an overall experience, it is probably impossible to beat the rooftop restaurant (Vertigo), I would even go as far as calling it Bangkok's best rooftop restaurant (with emphasis on rooftop).
3) So Sofitel: Could easily have won against many other hotels, but someone has to be last.
All of these 3 hotels score nicely in the culinary area, I don't see them at Bangkok's very top. They all can provide a nice experience but fall short to such heavy weights like the MO. I would also rank the Okura in front of it as it simply provided more lasting memories.

Pool:
Sofitel and Banyan Tree both provide a nice relaxing experience while the So Sofitel definitely wins view wise. Typical for the hotel, it is more pumping bass and neon light than relaxation.

All 3 are legitimate 5* properties and I wouldn't call any of them a nose wrinkling "Thailand 5*". Personally, I don't care that much about service as in "assistance to do random things". As far as I know, none of them provide anything like butler service and I can't comment much on concierges, bellhop and so on as I am more of a DIY guy. But all the service interactions I had were great with typical Thai hospitality. If you consider the Sofitels to be genuine 5* hotels, I don't think you will be disappointed. They are trading some blows but none of it is leagues ahead of either.

I think the Banyan Tree might be the winner if you are looking for a resort like experience in town. If I would not be allowed to leave the hotel for several days, this would probably be my favorite pick. I really like the So Sofitel as as it combines interesting style with a luxurious experience. As for the Sofitel, it does most things right in just being a very good hotel. It would probalby be my last choice if I were looking for more than "just" that. Then again, it would probably be my first choice if I had to find accomodation for anyone whose preferences I don't know.

Of course this is all my personal opinion and by no means meant to be an objective assessment, YMMV.