What is the most complex plant form?
That whole thing of being more complex is non-sense, you can't say that for animals and you can't say that for plants either. It all depends on how you look at it. Average number of cells? Size of genome? Size of the proteome? Adaptability to different environments? Mental abilities? You can't place human on the top of any of them (maybe we could argue about if you define it as mental abilities but that doesn't sound right) and you can't place amoeba at the bottom either.

Also, if you look at a phylogenetic tree, an organism is also not more complex by the number of nodes it has behind them (I've seen people thinking this means more evolved and associate that with more complex).