They're situational really. I'm in the habit of planning my inventory out meticulously - I don't always use all this crap, but I like having some of it around. Item sets make it easy!
Barrel Bombs - useful for breaking parts. If you're having trouble breaking something like a Diablos horn or whatever, give them a try. Their damage bypasses defenses.
Flashbombs - good for knocking flyers out of the sky or getting a quick breather to sharpen, set a trap, or carve a tail.
Sonic bombs - bring these to Diablos/Monoblos and throw them as it burrows - time it as the tail disappears under the sand. You'll get a nice big opening, even longer if it's tired. They're immune while raging though. Also works on Cephadrome.
Drugged meat - only really useful on Deviljho and Congalala who go after that shit like it's candy. Can speed up the fights a lot, try sleepy meat then using that super move thing that CB has on a weak point.
Dung bombs - you should be carrying these at all times. They get you out of pin attacks and cause monsters to flee areas - good for separating two in dual monster missions.
Poison/tranq knives - don't get used to these, it's rare they show up in the box and you can't make your own. But they are very powerful and it only takes a few hits to apply the status. The Kushala Daora caravan mission gives you tons of poison knives and some basic throwing knives - I brought extra toadstools and was able to keep him poisoned most of the fight. Which you pretty much need to do or else he's a pain in the ass.
Demondrug/armorskin - it's the same buff as the food skills and doesn't stack. But when I eat for attack up, I bring an armorskin along and vice versa. It lasts until you die so why not? Mega Demondrug is the same as Attack Up (L), and the regular one is equivalent to medium. But unless you're willing to farm khezu for weeks be careful what you bring those to, I stick to the regular ones.
Might/adamant seeds - give you a nice attack/defense boost and last for 5 minutes. When I'm 'serious' I bring these.
Might pills - give you a pretty huge attack boost for 20 seconds. This is what I use to murder the fuck out of Rajang while it's in a trap, I don't think anything else is threatening enough to bother.
Mosswine Jerky - secret best item. Buy tons of it from the guildhall vendor during a sale. It recovers all red health and cures Seregios-induced bleeding. In g-rank it's a free mega pot or more.
Combo books can be helpful to bring too. On 'serious' fights I bring an extra book and a 10 stack of honey - 10 more mega pots.
