On any battlefield, there will be tanks.  Huge masses of defense equipped with large guns, a tank is a frightful thing- you can’t take it down easily, and it will beat you up as you try.  Tanks tend to have fairly good stats, though they will often have relatively weak speed.  Because of this, Tanks can soak and dish out hits at the same time, sturdy sluggers that make an excellent mainstay for your team, with a chance to knock out virtually anything.  They often supplement this ability with one or two good support moves, which their durability gives them time to use.  At the same time, when it’s time to break something, a Tank is ready to dish out the pain as well, having access to a varity of moves that, while not necessarily the strongest in the game, will still lay on the hurt.

Tanks generally have most of their stats other than speed at close to the same numbers- and may have decent speed as well, depending.  If its defenses and HP base stats are all in the 85-110 range, it’s probably a tank- especially if it has one or both offenses in that range as well.  The Abilities tanks have are pretty grab-bag, and vary from thoroughly useless to amazingly useful- and include abilities seen on all the other kinds of pokemon with some frequency.  Some tanks will get more dangerous as they get more hurt, thanks to things like Overgrowth.  Some have excellent support to keep plowing onwards, like Ice Body.  And some have abilities that are just useful, like Levitate.  A few even have nasty offensive abilities like Moxie.

Some of the more stable tanks include Tyranitar, Venusaur, Empoleon, Weezing, Garchomp, Spiritomb, and Machamp.

Tanks are excellent pokemon to have on your team.  They are very straightforwards most of the time, and will form the meat-and-bread of your offense.  Their damage is reliable, their movepools can handle anything equally well, and their defenses are sturdy.  They tend not to have glaring weaknesses, making them harder to counteract, and will often keep chugging along no matter what is done to them.  Tanks like to rely on moves with good accuracy and power both, so ‘what if I miss’ or ‘what if I don’t do much damage’ is rarely a problem.

Tanks are terrible pokemon to have on your team.  Their unremarkability and lack of specialization means that they don’t have a particular job to do.  They are neither as fast as sweepers nor as durable as walls, and a lucky hit may be all it takes to knock them out of the fight.  They certainly need their share of lucky hits to break through most walls, and they simply aren’t fast enough to overcome a good strong sweeper before it pummels them to mush.  Overall, they’re just so... sameface.

Know your tanks and what they can handle.

Venusaur lacks good answers to flying pokemon, and while its Special Attack stat is good enough, its Attack is weak enough that it often goes unused- and leaves the thing easily stopped by any unpoisonable Special Wall.  Tyranitar is certainly nasty enough, but one strong Fighting attack will put it out of commission.  Empoleon is a decent threat to anything, and has good type coverage, but electric and ground attacks are very common- and mess it up completely.  Weezing has a limited array of attack types available to it, and will often fail against a fire-type that can knock it out fast enough.  Garchomp has a fatal 4x weakness to Ice, which everyone uses because Dragons.  Machamp is just another Fighting type, with the limited movepool and low speed that typically entails- and no good way to fix it.

Spiritomb is an almighty tank, with good stat numbers all around and a typing that leaves it with no weaknesses.  The ever-eerie purple swirl is immune to Fighting, Psychic, and Normal attacks, and can sit there lashing out with Dark Pulse and Ominous Wind all day long, using its Pressure to just eat through opposing PP.  But it also lacks a distinctive strength.  Anything without a weakness to Dark or Ghost attacks stands a chance against it, and if it gets unlucky on Ominous Wind, it -will- break after a short time.  It lacks ways to patch itself up, and can be worn down by a good Special Wall in no time.  And despite the lack of weaknesses, it has few resistances, meaning anything can beat it down over time.

If you want something stable and reliable, that will take a licking and keep on kicking, then pick a Tank.