Which marriage in Pride and Prejudice is the most succesful and which one is the least succesful?
Your question raises the interesting issue about Charlotte's marriage, and whether it is successful. It could be considered so in that Charlotte wants to be married ,and her husband, though silly, will indeed do that and support her financially. For Charlotte, not to be married would be the worst situation of all. Within the confines of her marriage, she is able to find the space to garden and, because her husband is often gone on his duties, space to relax as well. Putting up with her husband's silliness is not easy, but she adjusts, and that she is able to turn a lemon into a somewhat palatable lemonade does say something about marriage.
I would say that Jane and Bingley seem the happiest and of course, Elizabeth and Darcy. Jane and Bingley work it out sooner than Elizabeth and Darcy, but Jane has not the problems of pride and prejudice that the other two must work out for themselves.
The unhappiest couples by far have got to be Lydia and Wickham, two of the most self-absorbed people in all of literature. I would also say that the marriage of the Bennet's is none too successful, really; Mrs. Bennet is so scatterbrained, shallow and silly; her husband is intelligent but ineffectual.