Guai-Aid® 600mg "Ultra-Pure" Guaifenesin Gelatin Capsules (Be careful tho, they also come as tablets and only 400 mg. Attractive because less expensive but they do not work as well.)
I've tried them all, Mucinex, Kirkland brand, Geri-Care, liquid (too much high fructose corn syrup or artificial sweeteners and too much money! and so inconvenient)
Dye-Free, Sugar Free, Salicylate & Gluten Free -Ultra Pure - and in a capsule to get the meds in me more quickly. Best price found on Amazon for 524 caps or on guai-aid.com for 600 caps. About 0.12 cents a capsule when you buy in bulk. 100 capsule also available but more expensive of course. 0.22 cents a capsule.
I hope someone finds this useful. It took me years of experimentation with other products to find and love this one.