I've heard a lot of people say that learning any variant of Chinese first helps grealty with the other two that you mentioned. The universal reason is: you learn how to read Hanzi straight away, so learning Kanji and Hanja are very easy. Also, because Korean and Japanese have a lot of Sino-inspired vocabulary, you get a lot of cognates that make it easier to "decode" the other two languages.
It's sort of like learning Latin before learning Spanish and French. Sort of...
If you want easy... I can only guess, really, but I'd say Korean because if you go into N. Korea, you won't need to know any hanja at all and in S. Korea their usage of it is limited. Hangeul is all you need to survive
. For speaking, I couldn't say.