To me, the "best" is anything that works well and looks good in the end.
Sheets work just fine when machine quilting. And, if you get a sheet with a loose weave, they're just fine when hand quilting, too. Just make sure you wash it first to pre-shrink and soften the fabric.
If you aren't sure whether a particular sheet will give you problems when hand quilting - and assuming it's dirt cheap - buy it and do a test using the part you'd have to tear of anyway. If it's too hard to quilt through, put it aside for your next machine project.
I've learned so much from this forum. It's becoming more obvious by the day that the old rules don't necessarily apply anymore. Considering the price of fabric these days, I'm really glad about that. LOL
:-)