I am not aware of a commercial online website that has consistently provided a good variety of healthy, quality plants. If you are lucky and purchase shortly after they have received a new shipment of plants from their FL or asian plant farm suppliers then you will likely get healthy plants. If on the other hand you purchase when the plants have been in stock a few weeks, and the better ones already have shipped, it is likely you will be disappointed.
For the last six years I have purchased all of my plants from forum members on the planted aquariums forums such as Aquatic Plant Central, The Barr report, and The Planted Tank and have seldom been disappointed. Not only are the plants 'fresh' from someones tank but the variety is greater and I can find those rare and unusual plants (like Pogostemon erectus or Nymphaea micrantha) that the plant farms don't grow.
Dr. F&S is a commercial online website with the same caveats as mentioned above. Also along with hobbyists a lot of the sellers on Aquabid are actually retail stores, some of which have online websites of their own.......don't be fooled.
Nothing wrong with a commercially run source as long as you pick a good one. As far as which ones those would be, I couldn't say since I don't keep up with what they do. I do know that some actually have tanks and greenhouses that they grow their plants in, and there are several that basically just 'hold' the plants till they are sold. The latter can result in varying quality, but IMO a reputable company would offer a refund or alternate if they were faced with sending sub-par stock to a customer.
Getting plants from other hobbyists tends to be a good idea since you usually get more quantity, although I've noticed that some per-stem listings of more common plant species actually cost more than they would if you bought them in bunches from a retailer.
Typically the best are the guys who have several planted tanks and are selling off their extra growth. I'd be a little hesitant to get plants from someone who has never shipped before or isn't comfortable in doing so. The negative to getting plants from other hobbyists is they don't tend to have near as much selection as a plant retailer does. They do however tend to have more uncommon species as the farms tend to stick with a pretty generic list that only varies slightly by season.
One other big caveat to purchasing from retailers though is that many of their plants are grown above water, which means minimal/no algae, less chance of hitchhikers, but also tend to have a longer transition time to get them into submersed form, if applicable.