Why should I choose one particular congregation or denomination? Assuming that one believes in an almighty Lord, why should one choose a particular Christian congregation or denomination?

For example, why should one choose to serve God by helping the poor through the Salvation Army instead of doing so through the Opus Dei, or through Baptists, or through the Catholics?

As long as one lives in a moral fashion, why would God care that our money, faith, efforts and time go through a particular human congregation or another?

It seems to me that there is no particular reason for preferring one denomination over another, and most of them are quite similar, so maybe it's also OK to choose any church to be saved.

To be more specific, I don't care why I should choose denomination or congregation A over B, but why I should choose one at all.
To be honest, you have a good point. 

We're all here for the same reason, to glorify God and rejoice in his Son, our savior. For most intents and purposes, any denomination of Christianity will get you that much. 

As far as being part of a body of Christians at all, I believe this to be essential. Time alone is also necessary, but good Christian fellowship can go a long way; there are those to help you in your walk with faith, those to keep you on your walk, those to share your experiences with, and those to encourage you. This site could even be called a good site for Christian fellowship. The body of Christ is ever growing, and can a finger or a toe be a part of the body if it doesn't want anything to do with the rest of the fingers or toes, or they eyes or the nose?

As far as picking a denomination, this often happens because of very small, specific beliefs. I (being a Lutheran) selected my denomination because of how closely they keep to the original translation of the Bible (every pastor must learn Greek and Hebrew, and they cite the original languages often) and because of their focus on Grace Alone, Faith Alone, Scripture Alone. I needed to be constantly reminded of the fact that nothing I do can get me to heaven, and that God is the only one that has the ability to save me, and I need to take every chance I get to further my relationship with Him.