Sukhumvit Hospital is not bad.
They have some good doctors working there, Dr Prin for eyes is kind of famous, Dr Umaporn (urologist) works here as well as at Bumrungrad and Bangkok Hospital, Dr Chaiyasit (orthopedics) is a very experienced clinician.
Their young doctors are often very competent, too.
The physical therapy department is a bit unusual, but does have good points.
The hospital tends to be quite expensive, though.
The nearest cheaper place is Camillian Hospital. It's good, but has definitely less specialists than Sukhumvit Hospital.
And Sukhumvit Hospital is more accessible.
My experiences with St Louis Hospital are a mixed bag. But it's still the place I recommend most.