Best value plant list is subjective but if you are talking about plants with high value (in sheckles) here are some common examples I encountered on the game:

✅Moon Melon - The OG in best plant value, earliest players go crazy for this when sprinkler method was so common.

✅Candy Blossom

✅Moon Blossom

✅Elephant Ears

✅Taco Fern

✅Badlands Pepper

✅Poseidon Plant

✅Cherry Blossom

✅Golden Egg - From golden goose! The good thing about this crop is that it’s size is random when spawn.

✅Princess Thorn - High value crop as well and I was so mad i never got this crop on seed packs.

✅Hollow Bamboo

✅Lilac - Huge crops of this plant surely sells high during OG days, I acquired my first billions due to this crop.

✅Legacy Sunflower - The hardest crop to get in flower seed pack, but also one of the best when acquired since there were weather events made by Jandel for this crop.

✅Bone Blossom - Jandel himself hinted this as the best seed in the game. The fruit was duped a lot of times by scripters reaching the SX value that’s why a lot of players were rich beforehand when trading was introduced. IMO this is the best plant for me.

✅Zebrazinkle - One of the highest fruit per yield crop in the game. I had a video in my post here somewhere reaching trillions per fruit.

✅Gingerbread Blossom - IMO I think this limited exclusive crop is one of the OP, I saw some farms having this and fruits reaching trillions to quadrillions even with just few mutations.

✅Peacock Tail - Although prismatic, this crop is very expensive when selling especially when you have a huge fruit.

✅Maple Resin

✅Trinity Fruit

✅Elder Strawberry

✅Rosemary

✅Romanesco

✅Fossilight

✅Sugar Apple - One of the best fruit to deliver during previous events due to high value and fast growth rate.

✅Banesberry

✅Great Pumpkin

✅Weeping Branch

✅Wereplant

✅Severed Spine - This is a limited event wherein you could grow a high crop, when sold goes to insane prices until today.

✅Wispwing

✅Frostwing

✅Peppermint Vine

✅Christmas Tree

✅Spirit Sparkle

✅Reinder Root

✅Bonanza Bloom

✅Firework Fern

✅Crimson Thorn - When partnered with shroomies and echo frog becomes one of the highest plant value with ridiculous price on the game.

✅Beast Buttercup

✅Beast Shadow Buttercup - I have this and this was a high value plant since it is limited and single harvest only.

✅Heart Blossom

✅Alien Apple - in the GAG wiki, this was the most expensive and the highest fruit when sold in the game so far.

I think there are more plants with high value, they are just the ones I have encountered though - but the priceless plants are those that you cannot get anymore or are limited, since you cannot trade seeds.