My rules of thumb:
Science / Gold - settle in a city which has/will get lots of trade and make that a science/gold. (Try to avoid putting both in the same city.) Unless you're way behind in tech the single tech isn't worth it. (And even then, wait until you've discovered the one you're working on.)
Humanitarian - Can help a city with low food (i.e that one in the mountains which will have killer production) develop, otherwise wait until you've gotten all your cities started and use 'em for the population bonus.
Builder - Use them for a Wonder. However, there are cases where having half cost buildings in a city can be a logical alternative, i.e. a city on an island which has no production but will have massive trade, and that production powerhouse for pushing out spaceship parts.
Artist - Settle them in your captial or largest cities for the culture bonus. Yes, you can flip a city with them, but it's probably half-way around the world and not worth it. Defending flipped cities from both attack and normal culture flip is tough.
Leaders - Settle them in your production powerhouse and churn out elite units. Combine those elite units with two more (which don't even need to be veterans) and you've got an elite army. Or build triple-threat armies like tanks with scout, infiltration & blitz. Laugh when the AI uses them for a one-time veteran upgrade.