We have the numbers:
206:
Every non-zero number is a significant figure, and a 0 between two significant figures is itself a significant figure. Then, there are 3 significant figures.
26:
We have two non-zero numbers, so there are 2 significant figures.
190:
There are two non-zero numbers, but the extra zero is not between two significant figures, so this 0 is not significant. There are only 2 significant figures.
1.8:
There are 2 non-zero numbers, so it has 2 significant figures.
Answer:
206 has the most significant figures (3).