Couch Potatoes are the easiest to maintain plants that always give mega-snacks. You can get them from any of the mob creatures in Grizzleheim - Savarstaad Pass. They give a Fancy Yogurt (+50), Captain Cantaloupe (+45), or Golden Wheat Bread (+40) at elder harvest. They also give good high rank snacks and treasure cards at every harvest that can be sold for a lot of gold.
Like Seethe42 mentioned, Couch Potatoes are easy to manage and grant good mega snacks at Elder. I would also suggest getting Fish-on-a-Vine's. At Elder, they can give either Crab Apples or Crab Knight Cakes. Deadly Helephant Ears and Evil Magma Peas give a wide variety of Mega Snack packs. From my experiences, the Deadly Helephant Ears are a little easier to get if you're not looking to spend Crowns. Finally, Maelstrom Snapdragons have a chance at giving Mystic Dragonfruit at Elder, but are not guaranteed like the previous ones I mentioned.
my friend is a level 17 gardener. he swears by the Magma Peas...alas the magma peas are crowns only to buy. Still they give...Yogurt I think he said. He's been farming so much he's got like a hundred of that particular mega snack. 100 of another. Forget which snack though. The prickly bears if you don't mind spending gold vs crowns[I'd prefer to spend the gold frankly] as already mentioned give crab apples. couch potatoes give canteloupes...hang on there's a link I'll toss you. for this forum. this has been covered before.