My dog has dandruff, soap suggestions?

I have a 7 year old yellow lab. She is an allergy dog and we have had her since she was a puppy along with her sister from the same litter. Her sister perfectly healthy with a nice glossy coat that gets thick in the winter and sleek and glossy in the summer, where as Toffy has a very thick coat. She used to get bad rashes on her stomach from her allergys and we got her on meds for that. Now all she has left is a very thick, dry coat that won't even thin out in the summer and a lot of dandruff. My parents have spent a lot of money on her, even buying her special shampoo from the vet and stuff, but they are done with that since she no longer has her painful rash and all her fur is grown back on her stomach. So I was just wondering what some good brands of shampoo/conditioner would work for her. I was thinking of trying Main n' Tail or the Head and Shoulders shampoo/conditioner brands? For Maizy (her healthy sister), we just use dish soap to get the grease off her coat and some of our conditioner to make is soft and she looks awesome. But Toffy doesn't do so well, and she needs something stronger.