Rainbow Six Siege has been adding a ton of new operators continuously since the release of the game a few years ago, and that means there are a plethora of operators to choose from on both Attack and Defense. There are so many different things to think about when picking an operator in terms of who is best for which objective site, the map in question, what you're expecting your opponents to bring, etc.
To help narrow it down a little bit for you, here's a list of ten of the best operators on Defense. The operators chosen aren't in any particular order, and have been included because of how often they're going to be a solid choice; not by how effective they can be in specific circumstances:
Maestro is a very solid choice on most objective sites in the game. He's a 3-armor 1-speed operator, meaning he's much better suited towards playing an anchor as opposed to a roamer. His gadget further supports this playstyle. Maestro's Evil Eye cameras are entirely bulletproof (and melee-proof), and can only be destroyed by certain Attacker gadgets, like frag grenades or Sledge's Hammer, for example. His camera has thermal vision which is incredibly useful when defending against a team planting the defuser inside of a smoke-screen. His Evil Eye can also fire energy beams that don't do a lot of damage per beam, but the damage certainly stacks up over a few seconds.
All of the above is not even mentioning that he has a fantastic primary weapon that can be attached with an ACOG. Maestro is a really strong Defender in the right hands.
There are few objective sites in the game that don't benefit from a Jager's ADS device. Jager is a 3-speed 1-armor operator, and his gadget destroys any incoming enemy ordnance like grenades, Ash's breaching charge, and more. When placed well, his ADS can be a very difficult gadget to destroy, and can severely limit Attackers ability to use their own gadgets effectively. Jager's 3-speed and set-and-forget gadget makes him well-suited for roaming, and though he doesn't have the means to create his own rotation routes with an explosive or a shotgun, he makes up for that a bit by having a very good weapon. (Even without the ACOG)
Jager's partner-in-crime, Bandit is a great counter for the hard-breaching Attackers. He is another 3-speed 1-armor Defender, and his gadget is a Battery that electrifies reinforced walls, deployable shields, and barbed wire. The electric current prevents Attackers from using their own gadgets on those objects; like Thermite and Hibana, for example.
While most Attacking teams bring a Thatcher when they know they're going to have to breach a reinforced wall, a well-timed Bandit Battery can be placed as soon as he hears Thermite lay down his Exothermic charge. This keeps the Bandit Battery out of most of the danger in regards to Thatcher, but still allows Bandit to perform his role.
Bandit also comes equipped with C4, allowing him to make his own rotation routes in a pinch, though impact grenades or shotguns are generally preferred for the job.
Mira is, quite possibly, the operator that changed the game most when added. Mira is a 3-armor 1-speed operator, and her Black Mirrors allow for one-way windows through reinforced (or not reinforced) walls. This allows Defenders to stay informed as to what exactly is going on on the opposite side of the wall, letting them stay better prepared.
Many Mira's set up their Black Mirrors on a reinforced wall but leave the wall next to it unreinforced, allowing them to line up a shot from safety and peek towards the unreinforced wall while firing, usually giving them an easy kill.
While Mira is a 3-armor Defender without an ACOG, she has a super-useful and unique gadget and a weapon with a high rate of fire that more than makes up for it.
Valkyrie can be one of the most frustrating and difficult Defenders to play against. She's a 2-2 operator, and her Black Eye Cameras can be thrown just about anywhere in the map. She gets three of them, so with the right placement, she can cover a huge area of the map with cameras that can be pretty difficult for the Attackers to find. She can be an invaluable asset to roamers on her team, allowing them to have a ton of extra information they typically wouldn't, giving them the chance to act in situations where they normally would need to be more cautious.
Valkyrie's weapons are definitely lackluster, but as a support operator, she truly shines.
Mute is one of (if not the absolute best) counter-intel operators out there. Mute is another 2-2 operator, coming equipped with four Signal Jammers that disrupt enemy drones in the area, as well as other gadget's, like hard breacher's gadget's and Jackal's Visor.
They also stop Dokkaebi from having the chance to call your phone, provided you're in range or a Jammer. Well-placed Jammer's can really stop the Attackers from gaining useful intel into certain rooms, making Mute a great pick for just about any objective in the game.
On top of that, Mute has some serviceable weaponry, including a secondary sub-machine gun.
Lesion is one of the most difficult operators to catch off-guard. He's a 2-2 operator, and his gadget is a Gu Mine that continually gives him more to throw over time. These Gu Mines are very nearly invisible once on the ground, and when stepped on, they make a loud noise, deal damage over time, and stop the Attacker from sprinting.
The Gu Mine can be pulled out, but it leaves the victim vulnerable for a second or two to do so. Lesion can blanket a wide area with his Gu Mine's throughout a round, making it very difficult for Attackers to feel comfortable. Lesion also has a pretty decent primary weapon, allowing him to easily act on the intel he gathers.
An operator that might be surprising to see on this list, according to the most recent win-delta released by Ubisoft, Frost is one of the most under-picked Defenders and has the highest win-delta of all of the Defenders in the game. Frost is a 2-2 operator, and she has three Welcome Mats. These work essentially like bear traps, activating once stepped on, downing and pinning Attackers in place.
These Welcome Mats are generally placed underneath windows or on the top of staircases; in the places where they're the most difficult to see. However, Frost has been in the game for quite a long time now, so getting more creative with your Welcome Mat placement could net you a better success rate.
Frost also has a decent primary gun that, though it doesn't have a great rate of fire, stays pretty stable.
Yet another trap operator, Ela is a 3-speed 1-armor operator who has 3 Grzmot Mines that can be placed on any floor, wall, or ceiling. When an Attacker passes into the range of the mine, it detonates, concussing anybody in range. The concussion effect makes it very difficult for Attackers to accurately see what's going on and impedes their movement ability. This acts as a way to make gunfights swing in the Defender's favor, or at the very least, gives some intel for the team.
Ela has a very difficult-to-control primary weapon which is sometimes substituted for her shotgun, but she does have a pistol with a nice reflex sight.
Quite possibly the most banned Defender on this list in the Ranked game mode, Echo is a 3-armor 1-speed Defender and is one of the most powerful operators in the game. Echo has two Yokai Drones that hover around the floor or on the ceiling, going nearly invisible when doing the latter. These drones can spot enemies and fire ever-recharging concussion blasts that disorient Attackers for a short period of time. This concussion effect doesn't impede movement like Ela's Grzmot Mines, but it does prevent Attackers from planting the Defuser. This means an Echo in a safe location can easily prevent Attackers from accomplishing their objective if time is running short.
Along with his stellar gadget, Echo has a fantastic primary weapon and also a secondary sub-machine gun.