I forgot about celebi and co.. I was only watching pokemon above the UU. Sorry but, although they are good when making cores, amoongus, virzion, tangrowth are all under the uu cutoff. They all have niches like assault vest, spore, good typing but they ain't good enough like those fire types and water types. They're good I say as you pointed out, but not the best... While fire types and dragon types loop around the tier, there isn't a place for grass types..
Edit: Don't get me wrong, oddish, amoongus, venusuar, chesnaught are all in my list of favorite pokemon, and i do generally like grass types (and mudkipz!!) with oddish ranked at #3. But UU isn't the best place for grass types than say, RU where grass types rocks!