Personally, I think LostGar should run either 3 Prime, or 2 Prime and 1 Curse Gengar; The curse Gengar allows for damage counter movement, aiding with Compound Pains (after leveling up a Prime), which then in turns adds to the lost zone'ing, seeing Gengar prime's body sends anything originally KO'ed to the Lost Zone instead. (My own test build also had a good time against Vile[insert 2nd Pokémon name here] (aka any deck abusing Vileplume), get the plume active one way or the other, smack it instant KO with CurseGar, switch back to a prime, then send the Plume (that's at least 2 cards) to the Lost Zone by switching out to the prime and letting the body resolve before sending to the discard.
SF gengar on the other hand adds nothing to the deck imo, simply because SF gengar is more damage oriented, which isn't bad, but it therefore lacks the synergy with the prime version and/or the entire Lost World mechanic. Unless you want the deck to swing both ways, but that won't work either imo, seeing as it would take too much space to go for both ways; plus the recovery would make the deck very inconcistent as you'll endlessly recycle either the SF gengar or the prime one, as you're otherwise stuck with the one you teched in...
As for any of you who noticed I don't mention Gengar Lv X as an option, you guys are right; I think a Gengar Lv X is mandatory in any LostGar list, simply for the level down power, the extra HP, and the extra attack Compound Pain. Without Compaind Pain, gengar Prime would have no real ''damage dealing'' attack, which I consider a weakness. Without Level Down, Dialga G Lv X would eat Gengar's body, and by the means of level down you can assure at least 1 more pokémon in their deck to evntually Lost Zone if played correctly. The extra Hp (only 10 more compared to the prime) is rather a gimmick, but even that extra HP (even though slim) can end up being the difference between win or lose in the end. Therefore, and a lot of other reasons I think are self-explaining, I consider gengar Lv X mandatory in any gengar build, wether it be LostGar or not.
Obviously, 4 Seeker is needed, I don't think I need to explain why, now do I?
However, I read previously some one said 4 Twins is mandatory, but I care to differ; 3 Twins is my maximum to run; even though the Gengar player should never go for prizes in the first place of the LostGar variant, I find 4 Twins too space consuming, and 4 Twins maximize the chance of grabbing it early game; something where it won't be useful at all.
I've playtested many variants of LostGar, and so far all of my variants at least (ab)use 2 Cyrus' Conspiracy. Nabbing the energy is the main selling point, but fetching any supporter of choice is the money of the card. Of course, I also occaisionally nab myself a Poké Turn, but it depends on the build (yes, even LostGar variants without any SP trainers should (ab)use Cyrus imo, as the supporter and energy fetching are great assets to the deck).
The last thing I would like to point out is that I see a lot of people suggesting using Mr Mime and/or Noctowl to look at your opponent's hand. I strongly disagree; they use op valuable bench space, are easy snipe targets, and they don't add anything to the deck that Looker's Investigation/Judge can't do (yes I know Judge doesn't allow a peek at the hand, but after a Judge it's nonsense to really look at their hand anyways, just go for the Hurl into Darkness attack or wait until their hand is bigger). I personally run 2 to 3 Lookers, and I prefer them any day over a bench sitting tech that might/might not work. Did I also mention how it feels to start with a Hoot-Hoot/Mr Mime and for the rest nothing else to really go from? I didn't? Well, now I did, and see my point there? The best way to run LostGar, or at least they way I find playing best, is to run as less techs as possible, and be straight to the point. My own current version runs Gengar and Palkia G as the main lines, along with some SP techs, and Im still looking to slice out a few things.
I must, however, point out that running Palkia G Lv X also brings dire risks at times. Luxray is of course the number one fear for Palkia G, and Luxray's everywhere, and it will remain being everywhere, even when you play LostGar, and even when your entire meta goes LostGar (so to speak). A few friends of mine even teched in their own Luxray's along with their Palkia's, solely for the mirror match; even while you go for a prize that way, you reduce their options for Lost Zoning other than by KO or Hurl into Darkness'ing, while you can remain using Palkia's power and whatnot(and for the smart ones under you, it indeed becomes a game of who takes out the other one's Palkia first, or who has the fastest recovery running).
my 2 cents for now, Im curious to your reactions, and I'll continue to test various LostGar variants as well...!
''but James, you didn't mention [insert card name here, probably Mew prime and/or Absol G Lv X] at all!''
True, I either haven't tested with [insert card name here] yet, or I don't find the idea appealing at all. For a quick word on mew Prime though, I would never tech in more than 2; they are easy prize fodder and get rocking early on with little to no setup, but the LostGar player should always keep a double check on their opponent's prizes, even when not focussing on them themselves! Sacrificing 2 Mew primes is the biggest ''cost'' the LostGar player can afford, as all other times should be occupied with Lost Zoning things(without being removed from the field too soon themselves!).