Yi Quan is influenced by taichi, and is an impressive internal martial art so I've listed it here. Really it is a part of tai chi's evolution.
The founder learnt Sun style taichi, along with other martial arts and when he came round to creating his own system, he looked for ways to help his students become 'sung'.
It doesn't matter which martial art you do or health Qi Gong for that matter. In the end you are working towards cultivating a unified body which is capable of deep inner relaxation (fansung).
In Yiquan this takes full focus with a variety of standing Zhan Zhuang style exercises, silk reeling exerises and pushing hands forming the training, rather than the prescribed forms as in other taichi styles.
Movement when it comes is free form movement, not prescribed and so it involves a great deal if internal sensitivity and feeling.
The Zhan Zhuang standing exercises are very useful for opening the body and cultivating the Qi and so Yi Quan has a lot to offer as a health exercise.
It is also a complete high level martial art though, with a system in place to reach a fansung state so this is a very interesting system to consider following.
For more details on my meeting with a Yiquan Master see Xie Yong Guang