I think it depends upon what you are good at. I may choose Word over power point (because i have practiced at Word more). Sometimes I will snapshot the work done and use windows paint to do the arrows work (or adobe Photo shop) if I need the results in a picture format. May be we can have some good alternates from other researchers and Professors...
Witold, I also use WORD quite often to draw path models. Mplus is also quite nice, because you can alter the diagram. There may be better options, but for me these are generally sufficient.
I agree with the previous commentators that it depends what you are used to. I would add LaTeX to the list. as it has several packages for drawing graphs (graph in the math sense, as in nodes connected by edges). See http://tex.stackexchange.com/questions/57152/how-to-draw-graphs-in-latex
Witold, Daniel already added LeTeX to the list, which is quite nice, too. I have another candidate: We sometimes use Microsoft Visio, a diagramming and vector graphics application, if the graphic should be very accurate:
It is worth adding that packages for this type of thing allow you to draw "plates" (or boxes around sets of nodes) which is important for drawing the doodlebugs common for showing the models that are estimated in Bayesian packages like OpenBugs and STAN.
I made models in word but the formatting changes when you change computers and my professor did not like my figures. I later used EDraw software to draw models... It looks more professional.
What is better tool to 'draw' models including latent and observed constructs, PowerPoint, Word, something else?
For high level conceptual frameworks I use Microsoft PowerPoint. But for detailed research models inclusive hypotheses arrow-lines, I use Microsoft Visio. Also depending on the research & data analysis that you are conducting e.g. some of the statistical analytical tool inclusive measurement & structural model diagrams drawing capability like AMOS, SmartPLS etc.