Stallers aren't often used due to the fact that many players hate taking over 20 hours to complete a single battle. As a result, only one staller is usually ever seen in a legitimate party. And even then, whenever another player sees a Staller running around in the opponent's party, they will immediately turn off their system and look for another battle.

However, Stallers can be extremely useful in taking care of high danger foes, especially some like Mega Scizor or Mega Sharpedo (with speed boost).  Or even Mega Beedrill.

I myself have only used 3 Stallers: Dusknoir, Ferrothorn, and Registeel. (I run Snorlax offensively). However, I have gained experience in watching my friends breed and battle with different kinds of stallers. Here are some that can be a great addition to any party.

Registeel:

Registeel is very underused. His lack of offense is partially because of this. However, Registeel is underestimated quite vastly. Registeel possesses a base 150 on both his defense and special defense. A powerful potential staller or Wall in this regard, the only weakness Registeel has are the fact that his few weaknesses are commonly used elements. Other than that, Registeel has few weaknesses, and high resistance, and it's Clear Body ability further increases it's impregnability as a Wall.  As a Stall or Wall Registeel there are several approaches one can make.

Stealth Rocker: The Stealth Rocker Registeel is always put first in the party, Registeel's defenses, whatever his weaknesses, can always be guaranteed to last more than one turn if EV trained correctly. While obtaining the right IVs can be a pain since you have to repeatedly recapture Registeel over and over in order for this to work, Stealth Rocking can be a good way to inhibit other of your opponent's pokemon while taking your opponent for a long stall run. Seismic Toss is a key move to have thanks to Registeel's low offensive capabilities. However, it takes a LONG while to get a Registeel with Seismic Toss due to it's inabailability through tutors or TMs ever since the DS generations started. You would have to have a copy of Emerald in order to have the quickest path to a Seismic Toss Registeel.

Moves:

Stealthrock

Thunderwave

Seismic Toss

Earthquake.

Ultra Staller: This mode of Registeel depends on the sole purpose of just stalling your opponent out and whittling them down with statuses. The weakness of this however, is that it is useless against Poison Types and Steel, as it is solely dependent on the power of Toxic. Registeel is unable to present any effective offense against these. However, this strategy is far more obtainable than the Stealthrocker which depends on Seismic toss as it's main damager.

Held Item: Leftovers

Moves:

Toxic

Iron Defense

Amnesia

Earthquake.

The other Stallers have less varying strategies and will require less detailed explanations.

Poison Healer Gliscor. 

A Hidden ability of Gliscor, Poison Heal allows Gliscor to heal it's health by large amounts each turn as long as it remains poisoned. By hold a Toxic Orb, Gliscor gains an enormous health consumption advantage, being far more versatile in healing than any normal staller holding Leftovers. It allows Gliscor to use Substitute with very minimal health cost. Due to this, Gliscor gains a very long lasting power. However, Gliscor's excellent stalling power falls short when encountering any ice moves. Beware of Ice Clean Sweepers like Weavile or Ice stallers like Avalugg. The threat is minimized if the opponent uses Avalanch, allowing you to continuously set up Substitute before Avalugg can attack. But eventually it will come down to who has enough PP to continue this ultimate stall.

Item: (IMPORTANT) Toxic Orb

Ability: Poison Heal

Moves:

Earthquake

Toxic

Substitute

Stealth Rock

Dusclops.

With Eviolite, Dusclops is capable of becoming almost as much of a huge threat as Dusknoir, and even tankier to boot. However, due to the inability to hold Leftovers, in reality it can be more vulnerable. So in my recommendation, it is much more safe to stick with a Leftovers Dusknoir.

Item: Eviolite

Moves:

Toxic

Will-o-wisp

Shadow Sneak/Hex

Confuse Ray

Dusknoir

Dusknoir has a very straightforward strategy. Stick your opponent with pesky damage status like Will-o-wisp or Toxic and then inflict Confuse Ray. Confuse Ray will allow you some breathing room while you take the time to use Leftovers to heal.  It is recommended against physical foes that you use Will-o-Wisp, as it will drastically reduce your opponent's attack. Dusknoir is a good solid Stall to have thanks to his lack of weaknesses, and his immunity to heavy hitters like Mega Khangaskhan. In fact, since Khangaskan's only counter against ghosts tend to be Sucker Punch, as long as you don't attack and stick to your status moves, you can just watch the burn tear the mega away until it is nothing. I've used Dusknoirs to fight many Mega Khans around the US and Japanese players. However, some Khangaskhans use Outrage so be careful. However, as long as you burn him, and he doesn't have odor sleuth which would grant access to Power up Punch, this won't be much of a threat either.

Item: Leftovers

Moves: Same as above Dusclops

Weezing.

Weezing is a nice classic to have at your side. With poison moves like Toxic readily available, and Destiny Bond ready if you think the stall has finally breathed it's last turn, it's a ready made Suicide Style staller when necessary.. And a levitate ability further ensures that Weezing won't be taken out in one hit. My sister tends to use Weezing's Destiny Bond right when his health is near 0 to make full use of him as a stall (as Weezing is tough to one hit wonder without his ground type weakness.)

Ability: Levitate

Item: Black Sludge

Moves:

Toxic

Destiny Bond

Belch/Venoshock

Will-o-the wisp

Snorlax

Snorlax's vast amount of health reaching right into the 500s makes him even bulkier and more useful than Wailord. That, coupled with the ability Immunity, makes it almost like a counter against other stallers.  Snorlax is insanely versatile however, easily being able to be a Tank as well, granted you have the right nature and IVs. As a stalling Snorlax, when EV training you'll want to focus on Health primarily and then couple up with defense and special defense. Snorlax's greates advantage is special defense

Items: Leftovers

Moves:

Toxic

Sleep Talk

Rest

Giga Impact/Strength

Vespiquen

Ideally not the first one thinks of when they think of a Staller due to it's low maximum stats overall, With Defend Order and Heal Order, Vespiquen can be one of the most annoying stallers ever encountered. And thanks to the high critical rate of Attack Order and rather unusually high offensive stats for a stall,  she is capable of dealing quite the amount of extra damage.

Item: Leftovers/Bug Plate

Moves:

Heal Order

Defend Order

Attack Order

Destiny Bond.

Stallers are quite easy to find and train (especially since Toxic can be learned by nearly every pokemon in the pokedex) . However ones with evened out defensive stats like Ferrothorn and Registeel are very rare, so keep note there are some like Weezing that are weak against special attacks, or attacks if hit enough. If you wish for a Staller with minimal weaknesses and an equally powerful Defense and Special Defense, my recommendation is Dusknoir, Ferrothorn, Vespiquen, Registeel, and Defense Form Deoxys (Mind you, Defense Form is the only Uber allowed in the lower tiers, so don't expect this leniency with any other Uber legendary.)

There are plenty of other Stallers. If you wish to know about specific stallers that have been discussed or not discussed in this chapter, feel free to message me.