As Muslims we believe Allaah decides how good we really are.
He's given us various criteria to assess the level of goodness but these are just external indicators. Only Allaah knows the true extent to how good a person is or isn't.
O mankind! We have created you from a male and a female, and made you into nations and tribes, that you may know one another. Verily, the most honourable of you with Allah is that (believer) who has At-Taqwa[i.e. one of the Muttaqun (pious - see V.2:2)]. Verily, Allah is All-Knowing, All-Aware
(Interpretation of the meaning. The Noble Qur'an (49:13).
So the the best person in the sight of Allah is one who is most pious or God fearing. People are of various levels with the most God fearing at the top of the scale and the rest below them according to their level of piety.
If someone says, "I am pious", this is an indication that they are ignorant of what being pious really means and this person cannot be truly pious because being pious involves doing deeds of piety until it becomes a habit and not mere words.
The level of external or apparent piety can be assessed only via observing a person's actions and speech, character etc. This is the only external way of assessing a person's level of piety.
However the real piety is something that no person can truly see as the real piety is in the heart where only Allaah, The Most High can see. Only He can see how sincere the deeds, speech and character of a person really is.
If someone is a truly pious person Allaah will honour this person and make them beloved to the people without them desiring to be known. However if someone is not sincere and does things to seem great in the eyes of the people then Allaah will make their intentions apparent some way or the other.
Most of the time we see indicators of how "good" a person might be through the way they conduct themselves, the things they say and do which are considered "good" by religious or moral standards. If a person does "good actions" most of the time they are considered a "good" person based on actions that we can all see externally. But Allah alone knows what their intentions were in doing all these good deeds, whether they were done to show off to others or done purely for His sake. In this case He alone has this knowledge.
Not too long ago on this forum on this
thread I said, "I know that there are many good people out there among non Muslims and I give credit where it's due. But they are in the minority."
At a first glance someone who reads this might think "Who is he to judge people" and they might be right in thinking that however if you read the context of why I said that you may understand why I said it. It was said on a thread which was discussing how the media is working to discredit and insult Islam and Muslims. Then the topic of racism came up which still is very rampant all over the world. Add to that the various wars going on and some incidents where I have personally experienced racism and physical attacks and threats for a number of years and still see it every day. If you experienced what I've experienced and still see on a daily basis I think you might understand better why I made the statement.
My statement was also based on a Qur'anic verse which I have seen in various forms in several places:
You [true believers in Islamic Monotheism, and real followers of Prophet Muhammad
and his Sunnah(legal ways, etc.)] are the best of peoples ever raised up for mankind; you enjoin Al-Ma'ruf (i.e. Islamic Monotheism and all that Islam has ordained) and forbid Al-Munkar (polytheism, disbelief and all that Islam has forbidden), and you believe in Allah. And had the people of the Scripture (Jews and Christians) believed, it would have been better for them; among them are some who have faith, but most of them are Al-Fasiqun(disobedient to Allah - and rebellious against Allah's Command). (3:110)
It is not my place to judge people, except in cases where I have been a victim of oppression and have seen first hand a person who is inflicting their evil behaviour upon me as I have personally experienced.
Ultimately it is Allaah who decides who is good and who isn't. My statement was based on what Allaah has said Himself in His Final Revelation to mankind.
That being said, The Qur'an itself is not all negative. There's a lot of positive verses and teachings in there. It balances glad tidings with warnings so that people have a healthy outlook towards what is expected of them from Allaah and what they can expect from Him if they do good or evil.
The Qur'an is a primarily a book of guidance and mercy for mankind and focuses more on guiding people to the path that Allaah is pleased with for his creation and away from the path that He is not pleased with. It is full of stories of people of the past nations, stories of The Afterlife rewards for good and punishments for transgression, laws concerning the Permissible and Prohibited actions etc.
In it Allaah addressed all groups of mankind, Muslims AND Non Muslims so there's something for everyone in there.
Another thing to bear in mind as the person who replied to your questions suggested is that my experience might be completely different to another person. They might have the opposite view to me and might not have experienced what I've experienced which affects my perspective and influences my views on this matter.
However ultimately from an Islamic perspective being "good" is not a matter of personal opinion. It is based on criteria which are laid out in The Qu'ran and the sayings of Prophet Mohammed known as hadith. This is a universal set of criteria which all Muslims are expected to live up to. Some do and some don't. Some fall short and keep trying but at least there is a universal code available. This way there is no confusion and no one can make make up their own interpretation about what being "good" constitutes because it has already been done for us and the sources have been verified and largely agreed upon by the most knowledgeable people in terms of religious matters.