All of them have interesting things to do...both externally and internally...but the simplest is the best for our purposes I think. One of them is tenchinanage (6 or 7 I think)...so we're back to the beginning if we chose that one!
with tenchinage, you have to deal with both up and down at the same time, which is kinda hard for starting out. for starting out, either up or down would be ideal. easier with up than down. you can line up your body for dealing with the up better. with down, you need a decent "suit".
for the up part, start in hamni, right leg forward, not too deep. have someone push lightly to your right hip. open a path to the ground to from your right hip to your left foot with your mind. then have uke push your right shoulder and do the same thing. then push your right elbow. then right palm. you should not be moving, i.e. everything done "internally". so when uke grabs your right wrist, that same path should be there before the contact made. breath in and contract your stomach lightly so that a small tension built around your solar plexus and imagine your left hip joint and your right shoulder joints expand laterally and all your muscles expand longitudinally. the main point is to relax your body enough so that it can realign all the muscle and fascia tissues to do what need to be done. I'd say that's the starting point. at least, it's mine starting point. the first part is to absorb the "ki", then send it back; thus aiki. have not train my body for the send back yet; thus no aiki so far, only ai-arrrrggghhhh.....