For formal wear with dress, I recommend Club Mark. They can bring out the body figure nicely. There is one outlet in Vivo and another in Marina Square.

The Blum at UOB center in Raffles Place also carries a few formal suits. A lot of girls in big4 likes their design.

And the top brand for formal wear should be Armani, which is also highly recommended by several IB people. I am not sure whether Armani in Singapore sell formal suits. I think I do not have enough budget to buy either.

If you are looking at suits that are not extremely normal, Zara is also a good choice. Especially for matching pants.