Learn Finnish: Olet mitä esimerkillisin johtaja ja korvaamaton osa ryhmää. - You are an exceedingly exemplary leader and an irreplaceable part of the group.

This is the absolute superlative construction. By combining the word mitä with a superlative adjective (esimerkillisin) and a noun in the nominative case (johtaja), you create a phrase meaning an exceedingly exemplary leader or the most exemplary leader imaginable. It is more poetic and emphatic than just using erittäin (very).

Is esimerkillisin just the superlative of esimerkillinen?

Yes. The root noun is esimerkki (example). Adding the suffix -llinen turns it into the adjective esimerkillinen (exemplary). To make it a superlative, the ending changes to -sin, giving us esimerkillisin (most exemplary).

How is the word korvaamaton (irreplaceable) formed?

It is built from the verb korvata (to replace or compensate). By taking the verb stem (korvaa-) and adding the negative suffix -maton, you create an adjective that means unable to be replaced or irreplaceable. This is a very common way to form un-[verb]-able words in Finnish.

Why is ryhmää in the partitive case instead of using a location case like the elative (ryhmästä) for 'out of the group'?