For creating wordpress themes, people usually follow one of these two methods
Which is the better way of these (or anything other than these) on tackling the Wordpress theme creation?
It really depends on what your company/client requires. If they have specific requirements that cannot easily be met with a theme, you'll probably be better with option #1. If they are asking for a design submission, and are leaving the implementation/design up to you, go with option #2.
Some of the premium wordpress themes out there are quite good. Note that for either tool you'll be doing a mockup in Photoshop, so start with that and see what your company/client thinks.
You'd probably want to use an existing theme as a base for a new theme, or at least as a reference, for knowing which Wordpress Codex functions to call to retrieve data to use in your theme.
The HTML/CSS design is only half the solution - you still need to retrieve the data from WP to show.
It depends on what you're trying to accomplish. I lean and develop using Option 1, with using option 2 as a way to glean ideas from.
The main reason is that the photoshop mock-up, no matter how close you ask the designer to follow a existing template, "usually" is different in some fashion, so that by the time you get into the middle of the theme you find that the existing template you could modify doesn't accomplish everything that the client is asking for (unless the client is a relative, in which case you could say too bad).
The other reason is call scope creep. Meaning that the original scope that was presented has now grown past your theme. You'll then have to ask yourself if you can dive into someone elses code and figure out what they were trying to do and then see if you can hack it up enough to fulfill you clients new requirements or if you're better of developing from scratch and then when scope creep comes up, you know right where to go/do in order to meet their requirements.
Anyways, something to think about.
Use Sandbox to start your theme with, it is free and it gives you many classes to do a lot of the design tricks you see is great wordpress themes.
http://www.plaintxt.org/themes/sandbox/
Another great theme to start from would be Thesis: http://diythemes.com/thesis/
I always start from a theme that allows me to do things easier in the long run.