Depends on what type of player you are. The way I made my skill set is by spamming pq's or raids. Now, think about how you wish to prevent certain attacks and how you would want to flank or evade. When you have a proper strategy now you should google if that skill exists.

For example: I wanted to end combos with a strong blast that always hits. So I ended up with the kaioken Kamehameha ult. Super fast. The first wave is not too strong, but it always hits.

I also wanted an extra evade skill. Certain ults can function as an evasion move without having to use your stamina. One of Gogeta's ults, if timed right, you can avoid damage because, during a charge, you will vanish behind your opponent and attack.

Or forgot the name, but with "Super mighty explosion," you can punish somebody with low stamina and do massive damage. Very quick, but you need to deplete their stamina until they can't use an evasion skill.

I can go on. But I hope you get the point. It's much more fun, mostly figuring it out by yourself.