I've been to all three in a plethora of different room types and over many visits, Le Meridien wins overall in my opinion. For my cheapest available bookings, it is where I keep coming back.

BUT if you can book the TOWER, Sheraton is the place to go. Tower rooms are all recent reno and just as nice as LM, and location is important in Saigon, especially for a first-time leisure visitor. It will really impact your overall experience of the city.

If you choose to stay at LM, you'll be Grab-ing much more frequently and won't be coming back to the hotel for pit-stops much. Bring a daypack. Don't be afraid to Grab down to Binh Thanh, Independence Palace, or Bui Vien and walk from there.

Neither Sheraton non-Tower nor Ren are worth considering at all.