Good question. I think if you are looking to buy the mats, it may depend on what server you are on.
I would think something like Emetic Russula and Imp Stool for Ravage Stamina potions. Always use the highest tier water you can for more IP. This guide may help you determine what combos are good and then you can check prices: http://tamrielfoundry.com/crafting/alchemy/
If you want to farm some of the mats yourself, look for combos that use Water Hyacinth, as it's fairly common along waterways.
Also, make sure you've discovered the properties of as many plants as possible, as that also gives you a good chunk.