Formal:
* Welkom (most common and direct translation)
* Van harte welkom (literally "from the heart welcome") - more enthusiastic and heartfelt
Informal:
* Welkom (still used, but can sound a bit formal in casual settings)
* Welgekomen (more common in informal settings)
* Goeiedag (literally "good day" - used as a greeting and can also mean welcome)
Other options:
* Fijn u te zien (nice to see you)
* Leuk u te zien (nice to meet you)
It's important to remember that Flemish is very close to Dutch. So, you can also use the Dutch words for welcome, which are essentially the same: welkom and van harte welkom.
Tip: When in doubt, simply say "welkom." It's a safe and universally understood greeting in Flemish and Dutch.