You can think of this question negatively or positively. In other words, you might consider what is your worst trait and try to eliminate it, or think of your talents, and consider what other quality would best complement them and which characteristic you could best do without in order to make room for it. In either case, it is important to think first about what you want to achieve. What do you want to do with your life, and which aspect of your personality is holding you back from doing this? When you have identified the trait, you should be careful to ensure that you have not overlooked any positive aspect of it.
Even apparently negative characteristics can be valuable. Blaise Pascal wrote that all humanity's problems stem from our inability to sit quietly in a room alone. This may be true, but it is at least arguable that all humanity's achievements stem from the same source. The best world you can imagine is probably not a world full of people sitting quietly in solitary confinement. George Bernard Shaw took the opposite point of view when he remarked that the reasonable man adapts himself to the world, while the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man. Before you decide that you would like to be more patient, or more reasonable, or more anything, you should ensure that you are not losing a vital quality, one which might help you in the achievement of your objectives.
Finally, although this is an assignment which may appear to be of no more practical relevance than a question about the famous people from history you would invite to a dinner party, or what you would do with three wishes, it is worth considering that you actually can work to eliminate or at least minimize certain characteristics and develop others. You might, therefore, decide that you actually want to take practical action based on your assessment of the person you aspire to be.