I must have about 20 different carry ons :/ I could go through and tell you what's wrong with each one
I started with a decent enough large laptop bag which still works but with all the compartments, there's not very much actual room for larger items. It also gets heavy and has no wheels to roll it.
I think what's best for space is a simple holdall idea but something with a trolley/wheels on it BUT still light enough and the right size for carry on!
What do you use?