Just about any herb will more than pay for itself in terms of ROI in terms of time invested, so I am only going to focus on the top five. Keep in mind the best seed to plant for XP will always be the best you can plant that is at least three levels below your farming level. Anything above that will tend to get diseased and die far too often to be worthwhile.
Flowers:
Flowers are the easiest so I will get them out of the way first. Basically you have two options.
1. Limpwurt - These seeds sell for 241 today and are easy to come by. They harvest 3 roots (569 today for 1,707 or 1,466 in profit).
2. Marigolds - 63gp on the GE, and the harvested flower wells for 4,225.
Profit wise marigolds are the way to go. That is nearly 20k gp in 10 minutes of farming for planting just one patch.
Herbs
Like mentioned previously, never plant herb seeds that are within three levels of your current faring level (unless you are wearing an amulet of nature that is bound to that plot and are planning on staying logged in for at least 75 minutes). Here are the top 5 herbs to plant for profits:
1. Snapdragon: Today these are above 35k, which is on the high side. Try to pick them up around 31-32k. Even though this is the most expensive of all seeds, it offers the best return. 7.1 Snapdragons made into potions, and sold for 288k gives you a ROI of 113kgp. With more reasonable seed prices, I make about 150k per trip with these.
2. Ranarr: Today these are almost 20k each, high, but still very reasonable. Made into prayer potions, these would sell for 210k, giving a ROI of 110k. This is about the max you can expect for a round of ranarrs.
3. Toadflax: These seeds are 316gp. In this case, you are better off cleaning the herbs, adding them to a vial, and selling them if you do not have 80 herblore to make sara brews, so I will use the herb price. These will sell for about 94k, giving a return of nearly 90k, but at the expense of some herblore XP.
4. Kwuarm: These seeds are 3,282 each and made into super strength potions, they sell for 82k. ROI: 63k gp.
5. Harralander: These seeds are 36gp each and made into energy potions, they sell for a whopping 7k. ROI: 7k.
There are two honorable mentions here of seeds that didn't make the cut. Cadantine is good to sell if you sell the weeds, but loses money if made into super def potions, so it got skipped. Torstol offers incredible ROI, but the number of farmers who can farm it is so low I didn't thing it would be of worth to most of my readers, and those who can farm it know enough about power farming.
All seeds will give you a good return on your seed investment if you sell the herbs. Unless you never plan on training herblore, it is best to make your harvest into potions, because at some point you will need to buy those herbs back anyway. If you do choose that route however, make sure that you clean the herbs and add them to vials to make unfinished potions. In general, these sell well and for more than the cost of the added vial.
All sales figures were calculated using 7.1 as the average herbs per plot, that number includes a death rate of about 93%. The cost of vials and seconds are not included in the ROI.