bonega

Bona plus the augmentative -eg-, and you have the whole ladder without learning a single new root: malbonega awful, malbona bad, bona good, bonega excellent, with boneta for merely alright if you want it. Note that this is not a superlative. Plej bona means the best of a set; bonega just means very good indeed, no comparison implied. In speech bonege on its own is the standard cheerful acknowledgement, doing the job of English great or brilliant, and you will hear it in every Esperanto gathering within about five minutes.