This is a really popular question with the rising popularity of cutting machines and DIY personalized gifting. We highly recommend the multi-packs of neoprene (or similar material) options because they’re a great price for bulk buying, you can choose from a ton of different colors, and they’re easy to use with heat transfer vinyl.
If you’re looking for a slightly fancier gift, the stainless steel options can be personalized with HTV decoration as well.
For example, someone who is getting married might decorate stainless steel coolers for their bridal party and then get several bulk packages of neoprene products to be decorated and used for wedding guest favors.