it really depends on what you are looking for as almost all hotels serves some kind of a high tea. Are you planning on more of a continental or local flavor ?
Raffles is pretty good (be warned there are two locations in the hotel that serves High Tea, take the Tiffin room as to the Billard room) and of course, it comes with a price and dress code (smart casual).
Goodwood Park is good, more local items here
Four Seasons is alright with more international (sandwiches, canapes, etc)
Hilton's hi tea (think only weekends) is another decent choice as well as Intercontinental (they have two kinds in different venues, I had it at the lounge which isnt bad)
Mandarin (Mezzanine Cafe) Hotel has one that is pretty good (or was, last time I went there was 2 years back), their famous restaurant has hi tea as well so this one comes with a view which I like (both view and food)
Marina Mandarin used to have hi teas but not sure if they still have it nowadays after they reopened
Cafe Vienna (Royal Plaza on Scotts) used to have long queues for their lunch buffet, hi tea is supposed to be pretty good
Intercontinental has a good selection which includes sushi
Marriott's hi tea has quite a selection, busy and crowded as a result (may also due to location as quite a bit are locals)
Grand Hyatt has a hi tea which I like
When I visited Merchant Court Swissotel, I saw a sign for hi tea. If it is like the other Swissotel in Europe, their standard is not bad (good coffee and chocolate !)