Randy or Cassandra. I've actually never played with Phoebe, but it seems to have the least exciting difficulty curve.

Randy will hit you with chaos but occasionally you'll be showered with help, truly feels random. The difficulty still scales by colony wealth so it's manageable and it prevents me from over extending myself. It also feels more realistic that as your colony grows it'll attract more

Cassandra is good, but it feels a little more "on the rails" and I don't find myself in as dire of situations. I'll usually go with her if I'm trying to experiment more on the run.