To have a chunky scarf you will need a chunky yarn, maybe you have that already to go withnyour 6mm needles. Or you might hold 2 lengths of yarn together to create a chunkier yarn.
Any of the rib stitches you have said would be lovely, or make up your own k3, p5, k3, p2, k3, p5, k3 for instance, it adds a little more interest for you as the knitter as doing a plain rib throughout a straight scarf could become a little monotonous especially if it’s a long scarf.
Here’s a pattern with a little interest, it uses bigger needles than you are using which means it would be a bit less chunky than 5he ohoto shows but still lovely. There are a couple of different stitches which you can learn how to do with the videos provided in the pattern page. Of course you can ask here for help too.
Or perhaps this which again is a pattern for 8mm needle but you can still follow it with a 6mm needle and it will just be smaller. Again, a little interest for you as a learner knitter and a nice texture to give interest to the scarf.
There are videos for this too. Including how to read a chart and make the stitches.
Don’t hesitate to ask questions if you get stuck, whether it’s garter stitch, rib or any other stitch.