When we consider the best martial arts forms we are considering those martial arts kata that give the most benefit to the student. I usually recommend learning as many patterns and arts as possible, then working on the ones that the student prefers, although there can be oddities in this method. I also hold that one should learn entire arts, first taekwondo, then karate, then kung fu.
The kebons are good, basic kata taught in both karate styles and taekwondo styles. Though there are three to five of these introductory patterns, I don’t usually count them as forms because they are actually the ABCs of the martial arts.
The next batch of kata to consider would be the Taeguks from Tae kwon do. These are basic moves, a bit more advanced than the kebons, but not as advanced as the Okinawan Pinans (Japanese Heians). Though they take a few moves from the Pinan forms, they serve them up as straight block and counter techniques, no hidden throws or weapons disarms, and no real generation of internal energy.
After the taeguk patterns one should learn the Pinan forms from the Shotokan system, the Kyokushinkai system, and other Japanese martial styles. The Pinan kata are actually designed more for weapons defenses, though not many people know this. The idea here is that one learns the Taeguks for hand to hand combat, then moves into the Pinans for a basic understanding of weapons defenses, and the beginnings of chi eneergy generation.
After the Pinans I recommend the three forms from Pan Gai Noon, which is the base art of Uechi ryu Karate, and which are actually three extremely hard core kung fu forms. These three forms are sanchin, seisan, and sanseirui, though sanseirui is considered more of a show form. These three unique kata are specifically designed to generate internal energy.
Sanchin teaches a student to bolt the body/motor down to the ground. There are not a lot of moves in it, but the moves are perfectly designed for adapting hard energy to excellent self defense moves.
Sanchin may be the power form, but seisan is the technique kata. This form takes the power of sanchin and transfigures it into (probably) 13 specific self defense moves. These are all based on one specific move called wa uke, which is a circle block with a flesh tearing grab on the end.
So, Kebons to Taeguk to Pinans to Sanchin and seisan; taekwondo to karate to kung fu. This sequencing of martial arts forms provides the student with the absolute best and most complete arrangement of classical training possible. Other forms can and should be learned, but the heart of the art is really in this arrangement of art.
You find a lot more concerning the correct arrangementof form and art by visiting Monster Martial Arts. Evolution of Art might be of specific interest to classicists.