The geranium possibility on f36r gets a lot of votes - starting with Ethel Voynich (according to her list of plants discovered by VMS researcher Richard SantaColoma).
Nikki Phipps, author of The Bulb-o-licious Garden, explains why geranium leaves turn yellow here:
One of the most common causes for yellowing leaves is too much moisture or over-watering... Water or air temperature that is too cool can also result in geranium yellow leaves. Geraniums are a warm weather plant and they do not deal with cool weather well.
Other plant drawings bring more disagreement. The spectacular picture on f16v of the Voynich manuscript includes too much symbolic to get to conclusion that can be acceptable without knowing the author's intention. I came up with at least two possibilities that I haven't seen mentioned and many of those already proposed make a good case one way or the other.
The first proposal comes from image I accidentally saw while researching belladonna. It is in BNF Latin 9474 ( here ) - picture of Jacea Nigra also labeled flamette. The flame-name today belongs to totally different flower, but in early 16th century was associated apparently with Jacea.
Case can be made also that the drawing could be meant for Cyperus.
The Voynich plants with symbolic artistic appearance still keep their mystery locked. The hope is that the author's intentions will be made known one day when the text is explained. Until then the guessing game continues...