Being a huge fan of green ramp and fatties, I've run all three of those at some point or another, and my eventual choice for favourite ended up surprising me -
Sylvan Primordial. It turns out that eight mana is a lot more than seven, and it's rare that there's more than one troublesome noncreature permanent that you urgently need to deal with. The body is excellent defensively the turn he comes down, and he can continue to hold the fort for as long as it takes you to lock the game up, at which point he can start going to town. Perhaps as importantly, he's all upside - the forest might be an incidental bonus when you've already hit seven, but at least you're not giving anything back to your opponent.
Terastodon is better suited to a reanimator strategy, where you can cripple your opponent's early game by taking out a few of their lands, while adding, say, 12 power to your own board. So his inclusion should hinge on whether you plan to
Mannequin (or something) him out. If you're looking to hardcast a spell at eight mana, though, you're typically better off with
Craterhoof Behemoth, who often just ends the game, opponent's noncreature permanents be damned.
Woodfall Primus is the most pedestrian of the three, as while he wins points for durability, he's mostly a lower impact Sylvan Primordial when he hits the field.