In the self-help manual Pushing to the Front (1911) by Orison Swett Marden, at the start of Chapter XV ("What a good appearance will do"), I find this quotation:
I hold that gentleman to be the best dressed whose dress no one observes.—ANTHONY TROLLOPE.
English is not my native language and I cannot get the exact meaning of this quotation.
First off, this appears in an 1879 essay about the writer William Thackeray by Anthony Trollope. What it means is perhaps clearer in its original context:
I hold that gentleman to be the best dressed whose dress no one observes. I am not sure but that the same may be said of an author's written language. Only, where shall we find an example of such perfection? Always easy, always lucid, always correct, we may find them; but who is the writer, easy, lucid, and correct, who has not impregnated his writing with something of that personal flavour which we call mannerism?
I doubt I say it better, but Trollope is saying, in effect: if someone notices how you are dressed, you are not well dressed. If someone notices your prose style, there is something wrong with it. [In my personal experience this has always been the case: my clothes are over shabby, or (on rare occasion) too new-looking; I love to use extravagant figures of speech and fancy words, and so on. I have recognizable mannerisms.]
For Trollope, the best dressers and the best writers exhibit no mannerisms.
I am a cricket fan and it is popular in that field to say of both wicketkeepers and umpires that they have had a good game if you don't notice them. That is to say that you tend to only notice them if and when they make mistakes. The same is true of umpires and referees in most sports. Their jobs are so integral to the game that if they do them well it comes across as just the normal flow of the game and not anything noteworthy.
This expression is saying essentially the same thing, i.e. that our clothing is so integral to our social existence that those doing it best are those who are not noticed for doing it. If someone notices what you are wearing then it is noteworthy and, thus, not normal or proper and thus not the best it could be.
It depends on your own demeanour as to whether you agree with the sentiment or not. Personally, I find overly-flamboyant people to be annoying as they come across as craving attention, while others consider being "normal" to be extremely boring. It's quite possible that both are true simultaneously, because there is no one right way to be.
I hold
"I believe..."
that gentleman to be the best dressed
"...the gentleman who is best dressed..."
Note that "gentleman" has two possible meanings. The first is a man with good manners and good moral behaviour; and the second is a man born into an upper-class family. Or it might mean both. It was usual for upper-class people to think that only upper-class people could have good manners and good moral behaviour.
"Best dressed" means "the person with the best clothes". What does "best" mean here? We'll find out!
whose dress no one observes.
"... has clothes which do not stand out in a way which people notice."
The clothes he is wearing are "normal" or "average". If his clothes were unusual in some way, people would remember him as "the man who wore that shirt". His clothes are similar to everyone else's though.
Why does this matter? We need to remember that this man is upper-class, so he is rich. He can probably afford very good clothes. With his rich friends, he can wear those expensive, high-quality clothes, because all his rich friends are wearing clothes of the same quality. Wearing smart clothes is also a sign of respect, so if you wear scruffy clothes when you meet your friends, you are being disrespectful to your friends.
What happens when he goes out around town though? And more than that, what happens if he goes somewhere where everyone is poor? They can't afford high-quality clothes. Maybe they're lucky if they even have shoes. Everyone else there is equally poor, so they don't feel bad compared to each other. But a rich man walking through the town in his fine clothes will remind them of how poor they really are, and that will make them feel bad - especially parents who can't afford that for their children. He is noticed because his clothes stand out, and it feels to the poorer people that he is wearing those fine clothes to say to them "I'm better than you are".
So... To be respectful to the people who cannot afford the same quality of clothes, he should wear clothes which are not such high quality. Perhaps not as low-quality as the poorer people are wearing, but still not as high quality as he would wear with his friends. Then his clothes don't stand out, and it doesn't feel like he's showing off. And as a result, he is "better dressed" - and a better gentleman.
