What's the most valuable potion I can make using only the ingredients that can be grown in the Garden outside my Hearthfire house?

The most expensive potion would depend on your speech level, and since you said garden I'm guessing you're ruling out the house with the fish hatchery so that rules out salmon roe which nets the best potions moneywise I think.

Try these.

Blue Mountain Flower - Lavender - Wheat

Dragons Tongue - Fly Amanita - Mora Tapinella

Dragons Tongue - Snowberries - Purple Mountain Flower

Deathbell+ nightshade+ river betty= potion invaluable in combat

At 46 Alchemy

Blue Mountain Flower + Lavender + Wheat = 146g

Dragons Tongue + Fly Amanita + Mora Tapinella = 360g

Dragons Tongue + Snowberries + Purple Mountain Flower = 141g

Thanks for the reply i guess I will start growing Dragons Tongue, Fly Amanita, & Mora Tapinella in my Garden.