What's your favorite early-stage-design software and why?

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I'm definitely biased but I want to second @Julien's suggestion. I think Ladybug + Honeybee addresses your needs for early stages of design. Ladybug provides a great range of weather data analysis, and Honeybee supports connections to Radiance, Daysim, EnergyPlus and OpenStudio. We also developed several components for result visualization for Ladybug+Honeybee.

Since the development is on top of Grasshopper iterative modeling is pretty easy in comparison to other platforms. For more advanced analysis you can always export your model to OpenStudio for designing advanced systems.

Here is a presentation that we did at IBPSA-NYC last year. It should give you a better idea of the possibilities or you can watch these videos:

Part 1: https://vimeo.com/107501953 Part 2: https://vimeo.com/107502226

Though I have not used it extensively at all, you might be interested in checking out Rhino with Ladybug/honeybee as it could be a good candidate.

You could start by analyzing the climate using Ladybug, then using Honeybee to run E+ simulations with ideal loads on (and leveraging Grashopper's interface to change the shape of your building for example), and later on either model HVAC in honeybee or export to OpenStudio to finish the job. Honeybee also appears to have build in modules to create ideal shading scenarios, etc.

Rhino has a 90-day trial period, and Grashopper, Ladybug/Honeybee are free. There's a learning curve but its manageable.

@Mostapha Roudsari or @Chienman could probably tell you more about it.

I guess you would have to make a difference between "tools that allow you to quickly create models" (i.e. OpenStudio for EnergyPlus/Radiance and SU2RAD or Groundhog for pure Radiance ) and "tools that advice on what the design should be" (I am not aware of any of those that is currently in development or use... I heard of a few, though, some years ago)... I guess you will find more of the first kind than of the latter, and I think this applies to all design-related fields. For example, if you want to design a bridge, you make a proposal of how the steel might be arranged and then perform a structural analysis to see if your proposal was good (I understand that is how it works today).

I had this discution a few times in my last job. They always told me that the last advisor they had relied only on a highly educated common sense to make proposals and design alternatives. I believe that most of the people actually do that. However, building simulation tools allow checking if a design proposal is good or bad (just as the structural engineer designing a bridge) and also adding numbers to the analysis (i.e. everybody know that a lower WWR lowers the Solar Heat Gains and increases the insulation of the building... but, how much? what would be an appropriate WWR in this particular case?).

Long story short, I do not believe that any tool, nowadays, can "advice" very well on building design. However, there are tools, like the CLIMATE CONSULTANT that help getting an overview of the weather from a Weather File. This information is a very valuable input for your design process.

At least this would be my approach... first, study the weather; second, study the building requirements (internal loads, etc.); third, study the site available (size and shape, surroundings); fourth, use a tool that allows me quickly creating and evaluating design alternatives (i.e. OpenStudio and Groundhog).

Long post... sorry. Regards!