BLUE TONGUE SKINKS

I'm wondering what the best substrate for BTS's is? I'm using aspen snake bedding from Petco.
He loves to burrow in it and he seems very comfortable.

Aspen is a good bedding to use. Personally it is all about what you bts seems most happy with. If he seems to like Aspen then keep the Aspen.

I prefer cypress mulch and repti-bark.

I am using aspen for one of my Merauke, he seems to love it. It is great for burrowing and pretty cheap. The only complaint I have with it's that it's a bit dusty (But not enough to bother him apparently) and it is kind of messy. He manages to kick it everywhere and I always bring a bit out with him. But those are small complaints and overall I like it.

Then for my Northern I am trying out coconut mulch. She burrows in it well and it looks great. The pieces are big enough that they tend not to get everywhere and she can spit them out easier (I have noticed small bits of aspen can stick to food and they will eat it, but they are such small amounts that they won't do any harm). Overall I think I like the coconut mulch more, but it is also more expensive and difficult to get where I live so I might have switch over to aspen.

Mine both enjoy cypress mulch due to its ability to hold mid-high humidity levels.

My experience is as follows:

Aspen - sharp pieces, packs too tightly over time, and stays too damp underneath and does not radiate to the top properly.
TRex-Coconut - cubes of coconut are easy to burrow in and are very loose, but do not hold humidity properly. It is clean at first, but can be very dusty.
Repti-Bark - Best I have had so far. It is Fir tree bark and is loose enough to burrow, but heavy enough that crickets won't. It holds humidity very well, and usually comes prewashed so the dust is removed. It also has a high availability in stores.