To form a standing wave, two coherent waves must travel in opposite direction. But is it it necessary for them to have the same amplitude and no phase angle difference?
The fact that they are coherent means they have a constant phase difference. And from a constant phase difference it follows that their interference pattern will be stationary.
However, unless they have the same amplitude, the nodes of the "standing wave" will not be completely zero. Instead, you can think of the larger one as the sum of two waves. If we call the wave traveling to the right A, and the one traveling to the left B, then we can write $A=A_1 + A_2$ where $A_1$ has the same amplitude as $B$. Then $A_1$ and $B$ will form a standing wave pattern, and superposed on that pattern is the wave $A_2$. When $A=A_1$, $A_2=0$ and you have a perfect standing wave.