General:
* Plain: A flat, open area of land.
* Plateau: A flat, elevated area of land.
* Hilltop: The summit of a hill.
* Ridge: A long, narrow elevation of land.
Specific:
* Summit: The highest point of a mountain or hill.
* Peak: The pointed top of a mountain.
* Crest: The top of a wave or hill.
* Mesa: A flat-topped hill with steep sides.
The best antonym will depend on the specific meaning you're looking for. For example, if you're talking about the geographical feature of a ravine, "plain" or "plateau" might be the best choice. If you're talking about the depth of a ravine, "summit" or "peak" might be better.