Throughout the Kamen Rider franchise there have been many ultimate forms. While these look cool, a good chunk of them offer the rider an extreme amount of power with little to no noticable drawback such as Decade Complete.
Fourze's Cosmic States is the perfect example of how I think an ultimate form should work: It gives Gentaro an extreme amount of power but has a severe drawback that fits perfectly with Gentaro's personality.
My conditions for a good ultimate form:
1. The rider becomes extremely powerful (thats a given for any ultimate form).
2. It makes perfect use of the riders gadgets/gimmicks for the season.
3. It has an appropriate drawback based on the rider and in a sense sums up their personality.
Cosmic States works perfectly as an ulitmate form (no matter how lame the weakness seems to be). It is the culmination of Gentaro's relationships with other people and how he gains strength from the people around him.
It makes excellent use of Fourze's switches, even the one-shot switches, and even lets him mix and match them to create a variety of different moves and effects (though this was only used a few times). Though that fact was nicely offset by him not actually spamming Cosmic States at the beginning of the fight.
Its weakness may seem odd but it is also Gentaro's greatest strength: Friendship. The form cannot work if he loses someone's friendship. The switches synch with Gentaro's mental state so anything that clouds his mind or throws it into disaray will have a drastic effect on his ultimate form. Given who Gentaro is and the fact that the switches synch to his mind, the easiest way to stop Cosmic States is to destroy his friendships with people. Gentaro values his friendships more than anything so losing a friend would emotionally destroy him. When JK left the club he basically said that he wanted nothing more to do with Gentaro or the other club members, essentially severing his friendship with them all. Cosmic States reacted violently, electrocuting Gentaro and reverting him back to his basic state.
Fourze's cosmic state is by far my favorite ultimate form in terms of both pure power and how well it relates to Gentaro. Cosmic States speaks volumes about Gentaro's personality and says far more about him than I ever could.