Best fat loss plan to follow for a 6 week challenge

Fundamentally I think you'd first want to ask yourself if it's a good idea to take an approach where you're going to do something for 6 weeks that isn't necessarily representative of what you are going to for the other hundreds of weeks that occur after those 6.

Now I DO think there are SOME people, in some situations where trying to lose fat at a somewhat fast pace is warranted and possibly even preferable. But often times it's not a good idea. Ultimately it depends on the person, the approach, and what they're able to do when the aggressive period ends.

Typically people don't think this far into it, and after the "crash diet" is over they experience weight regain because they haven't actually figured out what to do, or how to eat, or how to develop better and more sustainable habits.

So to make a long story short, are you sure this is a good idea to begin with?

If you DO think it's a good idea, I'd select an approach that does some sort of habit development along the way so that you have a chance at integrating some of those concepts into the post-6-week-period.

I'd probably want the approach to consist of you eating almost entirely whole food sources with a lot of produce (heavy vegetable content for food volume and satiety) and barring any medical concerns I'd probably want you to be eating a pretty high protein intake such that each meal is built around primarily protein and large quantities of vegetables to promote satiety while keeping energy intake low.

From there you'd figure out how restrictive you need to be with dietary fat to cover essential fatty acid needs for general health purposes, and then finally how much (if any) additional carbohydrate to add in based on energy needs (to control rate of weight loss) and for adherence purposes.

So for example from a habit standpoint you might do something like add a serving of starchy carbs to X number of those meals per day to bring calories up as needed.

I'd also note that I'd take a similar approach even if you aren't trying to lose rapidly. The only difference would be the additional flexibility you'd have with the occasional indulgence, and/or the addition of greater portions of fat/carbohydrate to bring calories up a bit higher.