I'm going to suggest xingyi as well
but for *completely* different reasons that anyone else.
I train xingyi and periodically workout with aiki guys who're friends of mine. One thing xingyi trains better than any other system is down power via its signature fist--piquan or splitting fist. If it's not immediately clear why this is valuable for aiki, you might ask yourself why aikido's oft-ignored sword and jo work is thought to be a crucial element of success. Furthermore, aiki requires connected movement to be effective as does any of the Chinese internals (bagua's a good choice as well, but I think a little more problematic since both arts put so much emphasis on footwork).
Wing Chun has lots of sayings. If sayings won fights, WC would rule the world (presumably by talking its opponents to death).
Empirical evidence strongly suggests this has yet to happen.
--anerlich