First of all, we are all perfectly aware that aikido is an art and it’s impossible to rank its techniques in an absolute perfect indisputable way. However, doesn’t hurt trying to discuss the topic like they did in BJJ in this site: http://www.grapplearts.com/the-best-bjj-submissions-revealed/ I know that’s very subjective and depends on the kind of attack, but we can try instead of just saying “I don’t know, it depends” or “they are all equaly reliable”. Constructive discussion never hurts. So, what is, in your opinion, the Aikido technique that would not let you down if somebody really attack you outside the dojo?
I never did Aikido but lived with a housemate that did Shudokan Aikido. When they did multiple people attacks, were knackered and stressed and had to keep it simple, "irimi nage" was always a favourite. The version where the right arm is thrust forward and the outside of the forearm/elbow impacts the right side of the neck of the opponent rather than the one that involves lots of spinning and then clothes lines the opponent with the inside of the arm. I think there's a lot to value in slamming your forearm into someone's neck when the poop hits the fan.
Irimi nage AINEC. It's a footwork-based block, a setup, an entry, a strike, a takedown and an off balancing technique all in one....and it can still be modified a number of ways to offer variations on those themes.
I was going to say irimi-nage, too. Ikkyo is my 2nd. If it fails, then it becomes a good shove that buys you a little space and second of time to do something else. And that's a good thing.
Context dependent but I make liberal use of: Iriminage Irimitsuki Hijiate kokyunage Hijishime Udegarami-nage Ikkajo Kotegaeshi Rengyo
irime nage, ikkajo and sankajo (the last is very handy for dealing with stupid drunk mates at barbies who try and put a headlock on you to "see what you will do").