There are certain katas that are great for tournament and there are other katas that are great katas but terrible for tournament. The kata needs a certain amount of complexity that will demonstrate your technical ability. It also has what I call a presentability. Some katas just look better than others regardless of who performs them. The other thing is that some katas are better suited to certain body builds and attributes than others. If the kata has lots of kicks and you have no leg flexibility, this is an obvious one. On the other hand certain katas are better suited for larger or taller karateka than others and some for shorter faster students. Just because a kata is flashy and difficult, does not mean that it is well suited for you. If you are not able to consistently perform the moves without error, don't attempt to do this in tournament. A good example of this is the kata "Unsu". Unsu has a full 360 degree jump. An error made on this move and it doesn't matter how you did on the rest of the kata. Select 2 or 3 katas and focus on them. It is not necessary you know all katas forward, and backwards and out of sequence, but for these 2-3 katas you should.
The other thing is to watch what kata the other competitors are doing. If you know you will be close in the competition with another karateka then make sure you do not do the same kata as them. This is especially true if you believe that the other karateka can do the same kata better than you. Pick another kata that you do well that is equal or harder technically that you have a better chance in getting a higher score than your opponent. Another thing to consider is the style of the host school. Host schools are always going to judge katas of their own style more critically. On the other hand they may also give lower scores to katas that are done that are not a part of their style. This is a judgement call that you will have to make as to whether it would be to your advantage to pick a kata that is easily recognized or not. The disadvantage is the host school will always catch your mistakes whereas they may not on a kata they do not know and they will have to judge the kata on the other merits such as focus, balance, and presentation. The best advice I can give you is to watch the scores of those that perform kata before you and to watch what katas are being done. This will give you the best indication of what to do.
Be aware of the exact length and the degree of difficulty of the kata you are doing. Too short of a kata no matter how strong the performance will almost always result in a lower score. By the same token, doing too long of a kata leaves more chance that a move will be done in error decreasing your chances of doing a kata without mistakes. In addition, too long of a kata for many is too difficult to give 110% of power and focus for the entire duration of the kata with the karateka losing focus as they become tired toward the end. The degree of difficulty is also important in that even if a kata is done near perfect, if the degree of difficulty is not that high, the judges will not award as high of a score. A balance must be made for the individual performing the kata between length and difficulty so that a solid performance may be delivered.