The nature/ability and IV's are most viable in battle. If you're thinking about playing with this particular pokemon competitively, I would suggest you check out ... this is the suggested build for meganium on Smogon. They have a lot of standard build recommendations for pretty much every pokemon out there.
Take a look, and let me know if it helps give you any ideas of how you'd like to utilize him! : )
Pick one that's good for me
By the way i just read rng isn't possible for breeding in bw2 is that true?