Easiest to hardest, but also first encounter to last encounter, which is the same order, probably. I think we'll encounter multiple Oddish as we go, and we won't hit Kangaskhan until the Northern area.
Nidoran in this 'verse live in big families, I know from other-quest spoilers, so they're definitely harder than a solitary Oddish in terms of associated dangers, even if we aim to avoid fighting any Nidokings.
Of course, in an ideal world we run into a perfect Oddish right out the gate, toss it in a bag, find a lone Nidoran with a broken leg, and then an already-orphaned Kangaskhan. >.>
@Shin I'm actually quite fond of Tangela, but my reasons besides cuteness are speed and ease. I will be less concerned about both of those when we have our first pokemon and no two-week time limit to catch three pokemon across a jungle area, one of whom is rare and tough.