Helldivers™

This is by no means a definitive source of information, but I did find it useful in identifying some of the more commonly useful items:

JUST MY OWN OBSERVATIONS SO FAR:

General mission running:

• Solo = Avoid it if at all possible if you want to make any real progress. You can get way more done with a group... even just one more teammate can make a huge difference.
• Groups = The best players tend to play on the much higher difficulties. Best way to find them is to hot drop into another group rather than make your own (unless you can make a full group).

General weapon recommendations:

• The Scythe and Sickle are king of the PUG weapons for two specific reasons: they rely very little on ammo (so you don't have to carry your own supply drop or rely on others to) and are extremely accurate (to avoid TKs); good for almost any PUG composition you might join.
• The Breaker shotgun is probably the best starter weapon to begin with. Easy to aim and reasonable ammo suply. Works great against any faction. The bullet spread makes them exceptionally good for killing scouts before they can trigger the alarm.
• Level 6 missions are the point where it starts getting really challenging. The big difference: armored targets. At really high levels, you MUST have some "Anti-Tank" weapons in your group setup. Armored Piercing weapons are also recommended, but not required.
• The Justice rifle is commonly viewed as one of the best "all-rounders" in the game; chiefly because it also has some penetration capabilities (unfortunately you have to be lvl 18 to get it). A "cheaper" (in-game) alternative is the Paragon, but I believe that is in one of the DLCs; it is not quite as good, but shares some similarities.
• The Railgun is incredibly powerful against single targets, but sucks for general purpose. Not recommended at all for PUGs.
• The Arc Thrower "shotgun" is EXTREMELY powerful (and requires no ammo), but definitely NOT recommended for a full group unless you are VERY careful about TKs; ideal for solo play or only one teammate.

Support Weapon recmmendations (drop pods)

• Best AT weapons on the cheap: the Recoiless Rifle and EAT-17 (RPG). The RR is extremely effective when used in pairs (one person shoots and one carries the ammo pack to reload the shooter), but the problem is that if you lose either the launcher or ammo pack, you are screwed. The beauty of the EAT-17 is that it gives you access to an unlimited supply of 1-shot disposable launchers (have each member carry one if you want). On higher difficulties, this is EXTREMELY important. Both are also excellent for quickly destroying target objectives (faster & cheaper than other options).
• The "Guard Dog" personal drone is probably the single best counter to those annoying cyborg dogs on cyborg faction missions. Fully upgraded, it wrecks any of the smaller/faster enemies that like to rush you.
• The "Angel" healer drone is only okay-ish. At higher difficulties, it can't keep up with damage inflicted (best to avoid getting hit). A better way to keep your teammates alive is a fully upgraded REP-80 "healgun". A better way to keep you alive is a personal shield. The Angel is just kind of a jack-of-all-trades heal buff.
• HIGHLY recommend EVERYONE maxing out the UAV drop item. At the 3rd tier you can see research samples on the map. This will help you unlock everything faster.
• Fully upgraded gatling/railgun turrets are beastly defense options, however, I do not recommend deploying more than two at a time due to friendly fire concerns.. Can be rather fragile, so I highly recommend taking the REP-80 "healgun" to keep them functional. The manned turret is far less useful because it requires your full attention to be effective..

General vehicle recommendations:

• The Bastion tank destroyer is beastly; it is the single best (strongest & toughest) mobile AT option out there. However, it requires 2 crew and needs support from 2 more. Only use one at a time. DO NOT TAKE UNLESS YOU HAVE A FULL SQAUD.
• You will seldom go wrong with spamming tons of mechs, however, I don't recommend more than two at one time (best to keep two people to interact with stuff on the ground)
  • EXO-48 Obsidian (anti-personnel) - strongest in Lvl 5 missions or lower. ineffective against fully armored targets. Watch your ammo.
  • EXO-44 Walker (multirole) - fully upgraded, this is the most well rounded of the three mechs. it is very easy to burn through your 8 missiles, though; assuming they are fully upgraded, save them for armored targets (one/target should be enough)
  • EXO 51 Lumberer (anti-tank) - powerful gun but requires careful aim (i aim for the "feet" of my target), larger AT ammo supply than Walker, but flamer runs out of ammo FAST. I recommend ONLY using one at a time in a squad and ONLY if you need dedicated AT support. If your aim is good, its cannon absolutely wrecks big targets, though.
• The Hammer Motorcycle (aka BattleBike) does not suck as much as you might think, but is definitely NOT recommended for any but groups of two people due to camera limitations. With just two people, however, this is easily the fastest way to blow through missions if you are interested in speed runs... especially if you are farming research samples to unlock stuff (after lvl 6 missions, it gets harder to do this effectively).
• The APCs are extremely powerful IF fully crewed with 4 people, but avoid using them on Illuminate missions Level 6 or higher (fuck those shield wall projectors)... ignore me on this and it will only end in tears... trust me. Good for reasonably fast speed runs too.