Glad to see Prankster Gourgeist back up to discuss. The Pumpkin has been a personal favorite of mine since I first found it, and I'm eager to see it put to even better use. So, here's the take of a competitive novice on Prankster Gourgeist.
First thing that comes to mind is the obvious SubSeed. Unlike a number of other users of the strategy, Gourgeist actually has decent bulk, and probably won't be relying on too many non-status moves anyway, which alleviates the issue of its lower speed. It's defensive Typing also grants it a fair number of switch-ins, resisting a fair number of Special Attacking types such as Water and Electric that would prey on it's lower Special Defense, not to mention immunity of its own to Seeds and Spores.
Priority Pain Split gives it semi-reliable recovery as long as the opponent has a healthy team member left, and gives a weakened Gourgeist the option to cut down an enemy mon's health, which can in many cases be the difference between a check and a counter's reliability for a match up. Between this and SubSeeding, Gourgeist is annoyingly difficult to wear down passively. There's the priority status moves, with Will-o-Wisp stopping Physical Attackers dead and Toxic shitting on enemy Stallers.
Last, though not exclusive to Gourgeist, Leech Seed does offer an (admittedly very long) timer to Baton Pass teams, which take it with them to keep their boosts. One tiny advantage Gourgeist does have in that regard is that, unlike Whimsicott (which is what I tend to see most often among SubSeeders), it's bulk and typing means it doesn't have to fear Scolipede running Poison STAB, however rare that may actually be, and its Subs can usually avoid breaking from its attacks, meaning its safe to go straight to seeding it.
4 Atk Scolipede Megahorn vs. 252 HP / 252+ Def Gourgeist-Super: 82-97 (21.9 - 25.9%) -- possible 5HKO after Leftovers recovery
And if worst comes to worst, he's got Priority Destiny Bond for one last hurrah, taking down at least one enemy with him, even through Magic Bounce. Since Baton Pass Teams tend to rely on one particular "un-phazable" mon to hold the boosts and momentum, eliminating said member tends to throw a serious wrench in the strategy. While Bannette could do the same, Gourgeist allows the use of another Mega, which opens its usability up to a significantly greater number of teams.
The other candidates all have their own equally notable merit, but whereas they either add a new niche to the mon (Defog Hydreigon) or somewhat improve their current role (Volt Absorb's immunity is a boon, but Jellicent still won't enjoy strong Grass, Ghost and Dark moves, which is a problem with Knock Off abound), Prankster just makes Gourgeist work so perfectly in what's already probably its most effective roles.