I terms of the qualities of the main characters, there growth dosent always have to be measured in just power transformations but in actual character development. Vegeta I say has the most character arch throughout the series. He goes from a cold hearted villain to gradually becoming more of a good guy to going back to his evil roots to finally going full circle to becoming a good guy. Gohan to has a lot of growth, he goes from a very timid and gentle person to embracing his powers and become a well skilled warrior to the kind of person who sees violence as an improper excuse for answers to becoming a fine and excellent warrior at the end of the buu saga. Even Trunks has a bit of growth in the few sagas he appears in.
The argument for main character is fruitless as Goku is the main character of the entire series. One should know this by now, not to take away from Characters like gohan and Vegeta, they are just as important but they are not the main characters. Throughout the entire series, Goku is the main focus around everything. Gohan and Vegeta play their parts, but every saga essentially revolves around Goku in some manner, from Vegeta, to Freiza, to Cell and to Buu. The only real sensible case you make make for Gohan is his importance in the Cell Games. Not to take anything away from their characters, there just as important but they are sub-main characters, Goku is and always was the main protagonist in the series.
The argument for main character is fruitless as Goku is the main character of the entire series. One should know this by now, not to take away from Characters like gohan and Vegeta, they are just as important but they are not the main characters. Throughout the entire series, Goku is the main focus around everything. Gohan and Vegeta play their parts, but every saga essentially revolves around Goku in some manner, from Vegeta, to Freiza, to Cell and to Buu. The only real sensible case you make make for Gohan is his importance in the Cell Games. Not to take anything away from their characters, there just as important but they are sub-main characters, Goku is and always was the main protagonist in the series.