Which of them?

Analyzing a particular system will surely require detailed information about it so that it will be improved and developed further. Apparently, a person, team, or department who has the knowledge about the system will be approached for the purpose of inquiring or to gather data and for the system to be understood. Therefore, communication is essential for achieving these data—both for the client or user and the developer of the system.
In systems analysis, there are some identified techniques used in gathering data: Interviewing, Document review, Observation and the Use of Questionnaires. If I will be asked to categorize these techniques from what I am most comfy to make use of to the least one, it would be: Interviewing, Use of Questionnaires, Document Review, and lastly, Observation
I have ranked first the Interviewing scheme for the reason that it is what I believe the very efficient method if we want to make more clarification or we need to get a deeper explanation, or understanding about the system we are studying with. It may not seem that I am a chatty person as others might think of but actually I am just putting and using it at the proper time, place and person. As what I have experienced from my previous enrolled subjects which require visiting some establishments or companies, and involves a sort of interviewing with the responsible people, I realized that it was fun. I cannot say that I am a good conversationalist, but I can say that I have the confidence to talk with them in the best I can. Through Interviewing, we can possibly get even the most detailed information as we want to because we can make follow-up questions after the other and when we are being confused, we can ask until such time our queries are being responded. Use of Questionnaires is not that reliable for me. Unlike interviewing, having questionnaires is just limited.
Document Review will be strenuous, I think. You have to review, examine, and evaluate the lengthy document about a certain system. Although, the necessary information about the system is being written and presented on the document, there is no full assurance that you will completely comprehend the entire system right after reading it. It is likely to have questions after reviewing the document.
It would be difficult for me and for sure I will have a hard time if I am to use the Observation technique. Observing is very limited—it just depends on our five senses. If we have questions in us, it would be that we will or will never answer our own questions. There is also a big chance that if there is what we thought was the answer to our personal question, will be very far from the reality.
These techniques are all significant in gathering data specially in studying a system knowing that it is not that easy to fully understand them. Each of these techniques is applicable depending on a given situation. As far as I believe, they are all substantial in getting to know deeper in dealing with systems analysis.