Symbiosis. It's a nice lesson on the Prime Directive. Why the Federation should not cure one civilization's ills at the cost of another's.There also a drug lesson but thankfully we aren't hammered over the head with an anti-drug message.
"Beverly the prime directive is not just a set of rules. It is a philosophy, and a very correct one. History has proved again and again that whenever Mankind interferes with a less developed civilization, no matter how well intentioned that interference may be, the results are invariably disasterous." - Picard to Crusher
If I could suggest another,
Q Who? This introduces the Borg and also shows the Federation's flagship humbled by something out there stronger than they are and intent on destroying them. It demonstrates just how deadly exploring the galaxy can be and no haughty, puffed up attitude is strong enough. Plus there is
NO reset button. Okay, they are saved when Q recues them after Picard's begging. However eighteen crewmen are killed and nothing will bring them back.
"If you can't take a little bloody nose, maybe you ought to go back home and crawl under your bed. It's not safe out here. It's wonderous, with treasures to satiate desires both subtle and gross... but it's not for the timid." - Q to Picard.
What a great way to let viewers know that there is some excitement to come in the new series.