As far as which one would make the best Drought user, Heatran is the obvious pick. Even without Drought, it's a pretty consistent top 10 Pokemon in even the 1850 usage stats, so it's obviously already the best of the three in OU in general. It'd be able to take advantage of the same traits that currently make it so good in order to be a consistent Drought user, namely it's great typing, bulk, power, and movepool. The second best choice is a tough one, but I actually think I'd like for it to be Arcanine. Remember that a good trait in a weather user is its ability to switch repeatedly in and out when needed, and neither Arcanine nor Victini have the optimal typing for that. However, Arcanine does have a few advantages of its own. Intimidate is a major one, making it much easier for Arcanine to switch in and out when up against physically offensive Pokemon. Morning Sun is huge as well. Not only does it give Arcanine a form of healing (which the others lack), but it's boosted to 66% healing by its own ability to set up Sun. This would make Arcanine much better at switching in and out as much as necessary. Victini would still be a great Drought user, especially with a ridiculously powerful V-Create at its disposal. However, its Psychic typing makes it Pursuit weak, which is dangerous; Tyranitar is the main problem, switching in to change the weather and Pursuit trap Victini. Of course, I honestly think any Pokemon with Drought would easily make OU (Ninetales would be in PU if not for Drought), so any of the three would do nicely.