Far be it from me, a nobody, to tell an aikido yondan which aikido techniques are "best" for training aiki...
I would assume ALL and ANY technique would do, as long as the central elements of what makes it "aiki" are being trained???
The problem, as I see it, is that aikido waza are part jujitsu technique/part "aiki exercise". It wouldn't be a problem if it were simply treated as exercise, and not some bastardized jujitsu waza. So, perhaps the focus should be less on what "technique", and more on what is being exercised (or meant to be exercised!) in aikido waza.
Or looking at it another way... if one moved according to the principles of what makes it "aiki", then any technique becomes aikido waza, would it not?
To throw another spanner into the works... what is uke doing as part of the "aiki" interaction? Is uke simply a throwing dummy? Or is uke actively working those same principles - standing structure, upper and lower crosses, central pivot/equilibrium, all the while providing nage with an input force, with the view to taking advantage of a breach of those same principles in nage?
That would depend on how experienced your nage is.....after nage has an understanding of correct feeling uke can resist appropriately so the emphais is on developing a strong relaxed feeling instead of muscling uke.
What we call katate tori irimi nage (looks like sayu undo) is a very good technique to start with...start by teaching the principle of unbendable arm, then practice the technique with no resistance. Next demonstrate the technique with uke resisting logically so if nage loses correct feeling their outer stucture fails. As nage learns to depend on a low centered feeling instead of her arms....ki is developed.