I have stayed at all three of these Soi 11 based hotels over the last few months starting with several stays at Aloft, then a single-day stay at Fraser, and another multi-day stay at Mercure. I have also stayed at the Grand Swiss. I will gladly return to the Mercure anytime I decide I want to stay in Soi 11. As for Aloft / Fraser you may flip flop the rankings based on how important location/lower price is versus much bigger room/higher cost. As an aside, I found myself leaving Soi 11 in favor of Soi 4 often enough that I now just stay at the JW Marriott near Soi 4 / Nana. If you are looking for an experience closer to that of Pattaya you may want to consider Soi 4. Different topic I suppose.
Rank with comments and without considering cost:
By far my favorite hotel of all three. Centrally located in Soi 11 (across from Hillary 3 and very close to Aloft) this is a small swanky hotel (9 floors I think) with incredible service. Check-in is a breeze, Club Room is very well appointed and a great place to enjoy complimentary beer, wine, and very decent food throughout the day (breakfast too). I stayed in club room on 8th floor which was sufficiently large, very well decorated, had comfortable bedding and a great area for working in peace. The Hotel lobby is home to Alex Brasserie which has a menu that both your Thai girl and Farang taste buds can enjoy. Alex's semi outdoor patio bar is a great place to sit and people watch, or to catch the big game on the large screen tv. Busy at night. Lobby also serves as entrance to El Gaucho Argentinian Steakhouse which has a little more upscale dining. Overall feeling is upscale throughout the hotel and rooms which is reflected in service you get from staff.
TIP: Sign-up for the Le Club Accor when you arrive (it is free, okay it costs you about 10 minutes). I did this and was upgraded to the Club Room .
Fraser Suites $$$
Frasier offers condo-style rooms that are BIG modern, clean, and minimalistic, yet well equipped. The Fraser building is very tall and towers over Soi 11 which makes it look closer to the action (to the naked eye) than it really is. I stayed in the one bedroom apartment as an alternative to my frustrating stays at Aloft. There is a kitchenette with refrigerator, microwave, stove and a nice kitchen table. There is even a washing machine. A little much for a one night stay if you'r mongering. The bathroom is huge with a great tub if your into that with your girl. The "hotel" is otherwise fairly boring. There is a nice pool which I checked out and on that day was filled with old folks. There is also a very nice rooftop bar that you shouldn't miss if you stay there - you can seemingly view all of Bangkok from there. The Fraser is deceptively far from the hustle and bustle of Soi 11 which means your walking a bit to get to the action. Insanity Club is close by, but don't let the map fool you. Nothing is just outside the door. The lobby is nothing but a check-in desk - also boring. Staff - non-factor.
TIP: Just go the Mercure unless you have a family and need the space / amenities of Fraser.
The Aloft is a huge hotel centrally located on Soi 11, and owing to its lower price, has a younger patron demographic than the other two hotels. The large lobby is always humming with people coming and going, but despite having an ample bar, the drinking seems to happen elsewhere. Perhaps because the bar is fully inside and tucked away from any view of Soi 11. There is also a popular night club called Levels which is adjacent to the Aloft. I think there is some line-skipping privilege at Levels if you have a room key, but I also had a super hot Thai girl with me, and that might have been the reason we skipped the line. Note - you do not get to skip the paying of large cover charge. The two times I have been to Levels the action never quite looked like the videos you see on the website. Surprise. The pool is adequate with a small bar; however the real difference is the guest talent. The younger demographic means lots of hot young Farang girls laying around in tiny bikinis trying to cure their hangovers :). I have stayed at the Aloft on 5 separate occasions, and my negative experience has been consistent enough that I eventually decided to forgo the lower cost and benefit of earning Marriott/Starwood loyalty points in favor of other hotels. Recurring issues:
Despite super-attentive staff the registration averages 30-45 minutes - longer the closer you are to 3-4PM, but even in the late evening. Even if you are Platinum member with "special queue".
The Aloft Sukhumvit does not participate in Marriott/Starwood room upgrade programs. This is official policy, not my conjecture. (Four Points Inn on Soi 15 does participate fully). WiFi bandwidth (data throughput) is consistently horrible every time all the time. If enough people complain the front desk will have someone reset the router, but this is a temporary fix. I think the real issue is there are not enough connections to support all of the requests from hotel guests.
TIP: With the exception of a few very expensive suites all the rooms are the same with the exception of the floor/view. Don't fall for the slightly more expensive "upgrades". The rooms themselves are exactly the same.
Monger safely! - TGS