Choosing the appropriate healing item is one of the things that a player has to do while leveling. This guide is intended mostly for low and mid level players, who are propably working with limited amounts of zeny, and may not be aware of all the healing items available to choose from.

HP regened by healing items is increased by 2% for every point of VIT, and 10% for every level of the swordsman skill, Improve HP Recovery, and by 5% for every level of the alchemist skill, Potion Research. Note that when using aid potion, the target’s vit and imp hp recovery (if any) stacks with the alchemist’s potion research, as well as the 50% bonus from aid potion.

All healing items in RO can be assessed on three different statistics:
HP healed per zeny
HP healed per weight
HP healed per item used (imagine someone with 10k hp trying to use novice pots).
The first two of these are generally considered the main statistics to be considered.

Potions:
Item Avg Heal Heal/z Heal/wt
Potion, Red 55 1.45 7.86
Potion, Orange 125 0.82 12.50
Potion, Yellow 204.5 0.49 15.73
Potion, White 364.5 0.40 24.30

Red potions are only well suited for very low level players, because of the low healing per item used and poor heal/wt. Orange potions are commonly used by some players up into 6x levels (though see below, this isnt the smartest idea), with white potions being used pretty much exclusively at high levels.
Yellow potions have a less than favorable heal/wt and heal/z (they arent much cheaper than whites, but dont heal much more per weight than oranges).
At low-mid levels though, potions arent usually the best choice for hp regen. These food items sold by the npcs can be a better deal, especially when money is tight.

Foods:
Item Avg HealHeal/z Heal/wt
Veggies 18.5 1.54 9.25
Meat 86 2.26 5.73
Milk 31 1.63 10.33
Monster Feed 88 1.91 5.87
Tasty Fish 126.5 1.11 63.25

Meat stands out as far and away the cheapest healing item. Furthermore, the Spring Rabbit card, which is very cheap (50k) will increase healing from meat by 50%. Because of the high weight of meat, merchants can use meat the longest, because of thier cart - which is particularly helpful to them, because you usually need to spam healing items to level them effectively.
Milk is another great new-player healing item - it is cheaper and lighter than potatoes/carrots/apples, and heals more per use.
For mid level players, the standout item is Tasty Fish. These are sold in alberta, by the Chef Assistant, near the boat at the top of the docks. There is absolutely no reason to use orange potions when Tasty Fish are an option - fish are better in every way. They weight 2 each, so you can carry stunningly large amounts of them without going overweight.

Once you reach higher levels, where the healing per item of oranges/fish/etc make them impractical to use, your only option is pretty much white potions. If you have access to a ranked potter, ranked white potions are excellent, and white herbs are only 1k or so each, and most rankers get almost 100% on whites.
Mastala fruits, after 10.4, are so expensive that they arent worth using for most players - though on loki, the prices of ranked slim whites have risen so much that they arent much cheaper anymore. At high levels, low str classes dont have any other option than to use something like that.

My reccomendations over the course of a charachters life:
Novice Potions
Meat/Milk
Tasty Fish
White Potions

Move to the next one when the one you’re currently using is nolonger effective. We all have our own definitions of effective, based on our impatience and how much zeny we’re willing/able to spend.
Happy Potion Spamming.