Hi Midu
I think you are right that it all boils down to personal preference. This is art not science. There is no right or wrong. A picture you think is great might not be appreciated by the next guy - and vice versa.
My own personal preference is for something that strikes an emotional chord. Think Wow. For me that usually means simplicity. The picture focuses your attention on the main subject. It might be through colours, lines, harmony or contrasts. But the subject strikes that chord.
I've found over time that the pics that work best for me - but maybe not the next guy - are usually taken when I've found a background that sets off the subject. So I've tended to look for good backgrounds then waited to see what might wander into it - or live on it. This might be a goby that jumps onto the complex lines of a brain coral; or the lines and spines of a lion fish against an inky black background.
For me simplicity rules. But, like I say, it's art, not science. And the next guy might be a scientist.