Ok I have seen enough posts about Lilligant and its HP type. People are saying to use Fire or Rock. So why not compare them right here right now?
So let's say we use Hp Fire. The combo Grass/Fire is resited by Fire and Dragon. Dragon isn't common at all, but Fire definitely is! There are at least 5 common Fire type pokemon in the RU tier which I can think of right now. Let's start with the most obvious one: Entei. Entei can revenge you with Espeed if needed, or just laugh at your attacks and OHKO with Flare Blitz. Sure you need to have something asleep, but what is the first thing you send in on Lilligant? A sleepfodder or a Vital Spirit/Insomnia pokemon! Next pokemon: Moltres. Moltres quadrouple resists Grass, resists Fire and has a nice SpDef to back it up. Do I need to say more? Or do you want to hear that Moltres can Toxicstall, kill you with repeated STAB Fire Blasts or Air Slashes? Sure you want to beat Bouffalant but we forgot something there: he isn't a grass type, so Hp fire and Rock will deal the same amount of damage.
I would say use HP Rock. Steel is the only thing resisting it and how odd is it that Steels are really rare in the RU tier. The only few steel types who actually resist your attacks are also limited, because of there being many Steel/Rock or Steel/Ground types. Also I saw someone point out Durant, but it doesn't resist Rock due to the fact that Rock is Super Effective against Bug and it gets destroyed by HP Rock anyway (91.8% - 108.2% from +1 HP Rock so OHKO after SR). This leaves you with Ferroseed, Klingklang and Magneton who resist both Grass and Rock. Klingklang is so uncommon it isn't even funny, Magneton only can Thunder-Wave you to stop you and how can you have troubles with Ferroseed? It can't Leech Seed you and T-wave seems to be uncommon as well (even though it is a great move). So if we delete those as well from our list, what do we have left? As good as nothing.
So again, in my opinion, Hp Rock is the better option . The hits most of the common RU pokemon for at least neutral damage. SapSippers are annoying though, but you would only change the Hidden Power type to Fire for Sawsbuck, who is pretty frail anyway, as Bouffalant is just plain Normal type. Rock, on the other hand, gets better coverage all around because of only Ferroseed being the common pokemon to tank the hits.
(Also, I could be horribly mistaken, but don't be rough with trying to correct that. I'm just trying to make my point here, so please just tell me what I'm doing wrong instead of making insults.)
(Lol disclaimer and lol holy crap 487 words :D )
Edit: CRAP i forgot grass types >_<
Double Edit: oh nvm grass does not resist Rock.