There are a ton of Operators in Rainbow Six Siege, and Ubisoft has no plans of slowing down anytime soon. Almost every Operator can do a lot of good for their team, but some are going to be more consistently useful than others. There are a few Operators that are very situational, and this list is going to name Operators (in no particular order) that are consistently a boon to your team, and not necessarily Operators that are fantastic at what they do but only situationally useful.
Let's do Attackers first. No list like this is complete without Thermite, the original hard-breacher in Siege. While there are constantly new strategies being developed to combat Thermite's gadget, he still gets the job done pretty consistently, especially with some teammate support. A hard breacher in Siege is one of the most valuable assets to the team, and if they don't do their job right, it puts the Attackers at a severe disadvantage.
The other true hard-breacher in Siege, Hibana comes at the situation a little bit differently. Instead of a charge that she sets on the wall, she fires three sets of pellets that burn through the wall after a few seconds. This can help keep her a bit safer when she uses her gadget than Thermite, but it also means that Mute jammers can ensure that Hibana entirely wastes her gadget, as opposed to Thermite who can simply pick his charge back up when it's jammed.
Thatcher is everyone's best friend. Is there a Defender gadget causing your team problems? If it runs on electricity, Thatcher can take care of it. (And, let's face it, almost every single Defender gadget runs on electricity.) While he did recently receive a bit of a nerf in that his EMP grenades temporarily disable cameras instead of destroying them, he's still a real asset to the team and a hard counter to many different Defenders.
Sledge is another Operator that has a lot of value regardless of the map and regardless of the objective site. His sledgehammer is his own master key into just about any room in the map, and while he doesn't have quite the versatility of the next Operator on this list, he still has some solid weaponry, a very versatile and useful gadget, and is very easy for new players to pick up and play. (Not to mention he has access to frag grenades.)
Buck is the other Operator with a lot of soft-breaching capability. While his weapon is a bit more difficult to use than Sledge's, Buck has the possibility of soft-breaching areas above him as well as those on equal footing and below. This can make a huge difference when attacking different objective sites vertically. Plus, Buck also has the option to bring those ever-useful frag grenades.
The final Attacker on this list, Zofia is here because of the absolutely bonkers amount of utility she brings with her. Zofia's gadget brings impact grenades (making her the only Attacker with access to them) and concussion grenades. The impact grenades are pretty straightforward and make for a great breaching tool, especially vertically. Her concussion grenades detonate immediately when in proximity to a Defender, making them a great way to tell if a Defender is inside the room you fire it into, not to mention they give you an advantage in the ensuing gunfight, as well.
Bandit is the first Defender on this list, and his value cannot be overstated. Bandit is one of the best counters to an attacking hard-breacher, and at the same time, his battery can electrify barbed wire, making it an impassible obstacle for incoming drones. He's also a three-speed with a pretty solid weapon, which makes him a strong roamer.
Maestro is one of the Defenders with the coveted ACOG sight, and his gadget is both an invaluable source of intel and a fantastic tool for harassing Attackers. His cameras are bulletproof AND melee-proof, making only a select few Attackers capable of destroying it without their own gadget/utility. And this is all without mentioning that he has access to a fantastic primary weapon that allows him to hold the objective site very some serious swagger.
Jager is another 3-speed with a great weapon and a great gadget, much like Bandit. Jager's gadget destroys incoming ordnance like grenades (including Thatcher's EMP grenade) and certain gadget's, like Fuze's cluster bombs. Placed well, Jager's gadget can severely limit the Attacker's ability to use their utility, while Jager is free to roam around making use of his fantastic weapon.
Lesion is an unrivaled trapper. His Gu Mines are nigh-on invisible, requiring either the most observant Attackers the world's ever seen to spot them, or a Twitch Drone/Thatcher to destroy them. Lesion can continually fill the map with these mines, and if he can keep track of where they all are, he can gain a huge amount of intel on where Attackers are as soon as they hit step on/destroy one. These mines can also prevent Attackers from sprinting and planting the Defuser, making them a great way to time out a round.
Mira is, quite possibly, the character that changed the game the most since launch. Her addition completely changed how Defenders can choose to defend a site, and made certain methods of attack near impossible to accomplish. While Mira definitely takes quite a while to get good with and her mirrors cannot be used in every objective site, there are locations where she can be immensely useful outside of sites that she can't set up inside of. And, while her weapon doesn't have an ACOG, it is a headshot machine with that incredible rate of fire.
Valkyrie is another Defender that really changed the game in a drastic way, and her influence is likely going to felt as long as Siege sticks around. Her ability to stick cameras absolutely anywhere can give Defenders unprecedented intel throughout the map, and when well-placed, they're tough to spot unless the Attackers bring an IQ. Her weapon isn't the best, but it's definitely not useless, and the Deagle she brings is an absolute hammer.