No one can guess how much you need, in those temps you will see people in everything from 5 (or even 7mm at the lower end) to a bathing suit. Skinny people with low body fat will tend to be cooler. People who are used to warmer temps in general will also tend to be cooler. As said, the amount you are diving can make a difference too.
Personally at 30C I would be in a Lycra skin. At 26C I would be in a 1mm with my 3mm vest, or maybe my lavacore/sharkskin.
People often say you can't be too warm, you can let water in, etc. This is true to an extent, but having a heavier suit than needed can be a problem on the surface if you are in a hot sunny place. There's also a certain comfort and freedom for many people in wearing something lighter, and having less bulk to pack is nice too. (Light suits means I can pack 2, which means I will usually have a fairly dry one to put on.)