The Basics
Size Does Matter
Some slots are more equal than others. Some slots are bigger than others, and hold more than others, so here's how I identify each type of slot...

- Magazine Slot: Capable of holding standard magazines, clips, loose ammo, and very small firearms. Some melees can fit in this slot.

- Small (Sidearm) Slot: Capable of holding everything the Magazine slot can, and is large enough to fit a pistol sized firearm, including revolvers (for some reason, this also includes the Demoman's Grenade Launcher). Can also hold small melees.

- Medium (Primary) Slot: Capable of holding everything that the small slot can, and almost every firearm in the game, ranging from SMGs, Assault Rifles, Rifles, Shotguns, and short barrel LMGs. Also can hold large melees. This is also the first slot that can hold large magazines, including those from LMGs and drum magazines.

- Large (Special) Slot: Capable of holding (almost) every firearm in the game, including AMRs, large LMGs, Rocket & Grenade Launchers, and everything that every other slot can hold, excluding guns such as the gatling gun in Wurstworld.
Tactical
The default skin of quickbelts.
6 Small Slots
3 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 5 weapons, with 1 small slot to spare.
Classic
The good ol classic version of years past...
6 Small Slots
2 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 5 weapons.
Utility Belt
Very nice for sitting down!
8 Small Slots
2 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 6 weapons.
Desperado
For carrying all those shootin irons.
4 Small Slots
6 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 6 weapons, with 2 medium slots reserved for ammo being able to hold large magazines.
Pack Rat
Rat's packing heat.
12 Small Slots
2 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 8 weapons, however 4 of the small slots are reserved for pistols.
Tactical Mk. 2
Very tacti-cool!
9 Small Slots
2 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 6 weapons, with 1 small slot to spare.
Concealed Carry
For a real challenge!
4 Medium Slots (yeah, thats it)
You get 2 medium sized weapons and ammo, but not as hardcore as...
Small Pockets
For hardcore gamers who sweat G-Fuel. Cower to those who play Take and Hold with this!
2 Small Slots
You get a pistol with ammo at best, fear its users.
New TacticalTest
New and improved?
8 Magazine Slots
3 Small Slots
2 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 7 weapons, with 1 magazine slot to spare (not a lot you can do with that slot).
Tactical Centered
It's not really.
6 Magazine Slots
3 Small Slots
3 Medium Slots
2 Large Slots
Can be used (largest weapons prioritized) to hold 7 weapons.
RotR Quickbelt
Exclusive to RotR (for now).
9 Small Slots
1 Medium Slot
2 Large Slots
Can be used (largest weapons prioritized) to hold 6 weapons, however, it is wiser to use the slots for 3 medium weapons and use the rest of the 9 slots for holding ammo and special items due to the nature of RotR.
The "Best" Quickbelts
Best In Theory : Desperado
Desperado has one of the most Medium sized gun carrying capacities, the most common type of weapon in the game. 2nd place goes to Small Pockets.

Best for Limited Ammo : Pack Rat
Knowing limited ammo modes, you want as many slots for magazines as possible, which the Pack Rat provides with flying colors.

Best for Ergonomics : Tactical Centered
The easy to see, more natural positioning of the "Tactical" quickbelts offer a better visual configuration, and is on the higher end of storage for the weapons.
More to be added (eventually)!
Once Anton adds more quickbelt options, i'll add them to this list...