Preferably all of them, but that is up to you!
We can and prefer to create web sites that are compatible with all browser types and versions - if that's what you want. However, that includes certain limitations, as you can imagine. Just compare a 3 year old computer with any computer that is available on market today - You will quickly realize that they are completely incomparable.
It's the same with browser types. They grow smarter and better with time too, so if you want a web site that incorporates the latest scripts and features, then it will be compatible with the latest browsers. Don't worry though.. that has nothing to do with good design, only with scripting.
So, whatever your decision is, you will have a professional web site.