Basic Query: What’s the Best Software for Gear Simulation?

I'm curious what software people are using these days for gear simulation. I'm thinking specifically in the realms of either mechanical engineering – simulating the meshing of spur gears, helical gears, etc., seeing stress distributions and optimizing tooth profiles – or more along the lines of powertrain simulation and testing different gear ratios for performance characteristics in a vehicle.

there seem to be a lot of options out there, from dedicated FEA packages modified for gear analysis to specialized gear design software. What have you had success with, or conversely, what have you found to be overly complex or inaccurate for the specific tasks? what sort of learning curve should someone expect coming from a general CAD/FEA background? I'd appreciate any insights and recommendations.