More-or-less subjective TL;DR rating:
Sniper Pack >= Mod Courier > Weapon Pack#2 > Weapon Pack#1 > Shotgun Pack > Assault Pack > Big Bank >= Hotline Miami > Armored Transport
If you like hosting, consider picking up some of the mission DLCs, as DLC missions can be joined without owning said DLC.

**Gage Mod Courier**

Buffs for tons of weapons across the board. CAR-4, for example, goes from "pretty nice" to "godlike against literally everything". The mods are RNG-free, plentiful and free to attach.

**Sniper Pack**

All snipers pierce shields, enemies and thin walls. The anti-special ability (especially against Shields) is extremely potent. Unlike most of the DLC weapons, the sniper rifles are actually powerful. Comes with three rifles, Rattlesnake, R93 and Thanatos. Rattlesnake is weakest of the three, practically just a souped-up M308. R93 one-shots all normal cops no matter what, on headshots it one-shots all specials excluding Bulldozers.
Thanatos can two-shot Bulldozers with bodyshots. While incredibly powerful, it's only really effective against Bulldozers due to the horrible ammo efficiency.

**Weapon Pack #2**

Brings three light machine guns, RPK, KSP and Brenner. RPK is inferior to the others, and KSP has much higher magazine, ammo pool and rate of fire when compared to Brenner. LMGs in general have absurdly high ammo pools, mag sizes and rates of fire, and are thus extremely good at suppressing cops and holding chokepoints for days. Along with snipers they're the only weapons with no clear non-DLC equivalents. They suffer from -20% movement speed reduction when wielded, very low accuracy and a bad ammo efficiency, so on long and mobility-heavy missions they're often suboptimal. Also has several knifes as melee weapons. Melee is largely useless, with Pumping Iron, Trautman Knife can kill green cops in one hit, that's about the extent of their usefulness.

**Weapon Pack#1**

The main thing are the grenades. You get three for free for every heist and the ability to buy an asset case for anyone to use, which contains three grenades. The damage they deal isn't spectacular, they're most useful for area-of-denial and stunning Bulldozers. Bringing the server across the street on Firestarter day 1 is a situation which really benefits from a rain of grenades to keep the cops down. Signature .40 is a very potent Mastermind pistol if you have either the Technician headshot bonus or Enforcer +5% damage bonus. SpecOps SMG is a great SMG capable of one-shotting green cops. Eagle Heavy is a strong assault rifle which can two-shot tan cops. As an added bonus you can lock most weapons into single- or auto-fire, for extra accuracy or damage, respectively.

**Armored Transport**

The first heist DLC. Four very similar heists involving robbing transport trucks and a possible bonus day 2 Train Heist. Train Heist can technically be stealthed, although it relies more on RNG and glitches than skill to complete without alarms. Comes with two extremely concealable weapons, Gruber Kurtz pistol and Swedish-K SMG, the latter being the most concealable secondary available.

**Big Bank**

A polished, long, classy heist. While a great experience, the DLC is needed only if you want to host the map. The price is very steep for just that, unless you really like hosting. Comes with Falcon rifle, which happens to be the most concealable rifle which can two-shot tans without the Technician headshot bonus.

**Hotline Miami**
A great two-day heist. Includes a ton of more-or-less interesting melee weapons and three SMGs. The Uzi is very strong, as it has high damage, stability, accuracy and ammo pool, basically a secondary CAR-4. The DLC, unsurprisingly, includes four Hotline masks.

**Shotgun Pack**

New shotguns and ammo. The shotguns themselves aren't anything to brag about, as they are all outclassed by Locomotive 12G and Judge. M1014 is great with HE ammo and Sweeper is kinda comparable to Locomotive on Overkill and below. The new ammunition types are 000 Buckshot, HE Rounds, Flechette and Slugs. 000 boosts the damage a bit, HE rounds stun enemies, Flechette increases range and Slugs work as makeshift snipers, being able to penetrate Shields and walls. HE ammo is great for crowd control, and can lockdown bulldozers easily. The other ammo types have some niche uses, like 000 allowing a silenced Locomotive to one-shot tan cops and Flechette giving practically free range to Judge. The pack has many melee weapons, which are all outclassed by the community baseball bat. The KLAS shovel is still as hilarious as ever.

**Assault Pack**
New weapons, three assault rifles and a grenade launcher. The assault rifles are okay, Clarion is a direct contender to CAR-4. The grenade launcher has very small area of effect and it's very ammo inefficient, so without Fully Loaded ace it's more of a backup plan than a real damage dealer. The assault rifles are very similar to the existing ones, so if the GL40 doesn't interest you the pack has very little to offer.