For me, I'd pick ki and haki
Ki from what I understand has no restrictions, unlike nen and alchemy, (minus the user's body determining how much ki they can use, but you can increase that limit through training)
And for me, haki is my second choice because color of observation, if trained enough, can let you see into the future for a short time and color of arms imo is probably more versatile than ki is...perfect offense and defense