Let's get this out of the way right off the bat: Rainbow Six Siege is a very situational game, and it's very hard to quantify exactly how useful an operator or a tactic is going to be until the round starts. (Or, at least until the preparation phase.) With that said, there are operators in Siege that are going to be useful in a wider variety of situations than others. This list ranks those operators (Attackers and then Defenders) who are useful more often near the top and puts operators who are much more situational towards the bottom. The list has been made from a combination of personal experience and general player opinions. For newer players, you might want to work towards unlocking the more useful operators first, and for the experienced players out there, maybe you'll learn something new! With that said, if you're having success with an operator that's lower on this list, don't let this list stop you!
48 THERMITE (S-TIER)
Most objectives in Rainbow Six Siege are going to be a million times easier to attack if the attacking team has a hard breacher. Thermite is the original hard breacher in Siege, and despite needing to physically attach his gadget onto the wall he wants to breach, he's still a fantastic, and often critical operator to have.
47 HIBANA (S-TIER)
Hibana is the other true hard breacher in Siege, added shortly after launch. Her gadget allows her to open reinforced walls from a distance, and despite having a gun that can be a bit difficult to use because of its low magazine capacity, she remains another great operator pick.
46 THATCHER (S-TIER)
The usually-necessary companion to a hard breacher, Thatcher's gadget allows him to destroy/disrupt any enemy gadget that requires power to run. This includes the often-used Defender gadgets that foil hard breachers, like Bandit's battery, Kaid's electroclaw, and Mute's jammer.
45 MAVERICK (A-TIER)
Maverick is the only other (almost) hard-breacher in Siege. While his gadget can make a hole in a reinforced wall big enough to travel through, it's going to take a bit of time to accomplish, leaving him exposed for quite a while. It's often necessary to have a partner with Maverick watching his back. However, his gadget is also great for other things, like its ability to open unreinforced hatches swiftly, and its ability to destroy Maestro's evil eyes.
44 BUCK (A-TIER)
Buck is probably the best operator for vertical play in all of Siege. His ability to open both floors and ceilings makes him a huge problem for anchors in certain objectives, and on a well-coordinated team, his ability to push anchors out of the spots of relative safety is a difficult thing to counter, provided the enemy roamers are taken care of.
43 SLEDGE (A-TIER)
Sledge is another fantastic choice for soft-breaching/vertical play, and while he can't open ceilings like Buck can, Sledge can use his hammer many more times than Buck can fire his shotgun. If you're certain the objective site is going to be below you, Sledge is arguably a better choice than Buck.
42 ZOFIA (A-TIER)
Zofia has an astronomical amount of utility and versatility, making her a great choice for just about any objective in the game. Her impact grenades allow for incredibly quick soft breaching capability (including vertical play) and her concussion grenades are a great substitute for a drone if you don't have any left, since they explode immediately when near an enemy.
41 ASH (A-TIER)
Everyone's worst nightmare to have running at you, Ash is the primary entry-fragger in Siege, meaning she's usually the operator entering the building first and (hopefully) making quick work of a surprised roamer or two. While not quite as great as Zofia's, Ash's gadget is definitely good, and despite the recent removal of the ACOG from her primary weapon, it's still a beast of a gun.
40 TWITCH (A-TIER)
Despite constant nerfs to her gadget, her drone is still relatively effective at what it's there for: taking out enemy gadgets. While they're nowhere near as effective as they once were, they can still be a good benefit to your team. But where Twitch really shines is her weapon, the F2, which is one of the best weapons in the game.
39 IQ (A-TIER)
IQ is a difficult operator to rank, simply because of how much the enemy team can affect her usefulness. Her gadget allows her to see any enemy gadget that runs on electricity through walls, meaning she shouldn't ever be surprised by most of the gadgets in the game. However, the loss of her frag grenades really hurt her ability to destroy enemy gadgets around corners. That being said, in the current trap-meta we have in Siege, IQ is the perfect solution.
38 CAPITAO (B-TIER)
Capitao is another operator that has a huge amount of utility available to them, what with his crossbow that can either smoke an area or cover it in flames. The only reason he isn't quite in the A-Tier is because of his somewhat lackluster weaponry.
37 JACKAL (B-TIER)
Jackal is one of the best roam-clearing operators out there. His ability to locate and track down any Defender with ease (besides Caveira) is great, and he has some fantastic weaponry. He does, however, require a solid amount of map knowledge to use to his full potential.
36 YING (B-TIER)
If stun effects from flash grenades and Ying's candela worked with any consistency in Siege, Ying would be much higher on this list. However, despite the inconsistency with her gadget, Ying remains a solid choice, because when her gadget works as it should, it's supremely easy to walk into a room full of blinded enemies and get some easy kills.
35 BLACKBEARD (B-TIER)
Blackbeard is another operator that has been the target of quite a few nerfs, albeit many of them were warranted. Though he's a shadow of his former self, Blackbeard is still a real pain to get into a gunfight with, because in any gunfight of equal footing, Blackbeard will come out the victor due to his rifle shield.
34 MONTAGNE (B-TIER)
Montagne is an Attacker that can, on occasion, be quite literally unkillable. His full-body shield allows him to be untouchable by anything but smoke and traps, so there are certain corners he can wedge himself into that don't allow Defenders to do anything at all about him, until Montagne decides differently. With a well-coordinated team, Montagne can be a true force of nature; but without a well-coordinated team, he's all but useless.
33 NOMAD (B-TIER)
Nomad is, perhaps, the operator that can give the enemy roamers the biggest headache. Her airjabs can completely cut off roamers rotation paths, essentially trapping the roamers into an area unless they want to give themselves away. Map knowledge can really make Nomad a difficult operator to deal with, but without the proper map knowledge, her airjabs may be completely wasted.
32 GRIDLOCK (B-TIER)
The only 3-armor Attacker without the option of having a shield, Gridlock more than makes up for it with very good weaponry, including a secondary shotgun that gives her breaching capabilities, and a gadget that allows her to rival Nomad's ability to cover the flank. If you're aren't a fan of Nomad's weapons, Gridlock is a good operator to give a chance.
31 DOKKAEBI (B-TIER)
Dokkaebi would be a lot higher rated if she had a better primary weapon at her disposal. But as it is, Dokkaebi is an operator with a fantastic gadget allowing you to get an unrivaled amount of intel on the enemy team, only lacking in her fragging capabilities. If you happen to be a fan of using semi-automatic weapons in Siege, she's definitely worth giving a chance.
30 NOKK (C-TIER)
While Nokk's gadget can be difficult to use because of the limited time it can be active, when it works, it really works. It allows her to move almost silently and invisible to cameras, giving her ample opportunity to catch enemies completely unaware, though her lack of an ACOG can definitely be a problem, sometimes.
29 LION (C-TIER)
Lion is an operator that used to be wickedly overpowered and has been toned down drastically in order to put him where he probably should be. Lion can be a good choice, and can very much turn the tide in your favor, but in order to make him really make a difference, it requires an incredible amount of coordination with your team and the right situation given to you by the defenders. Most of the time, his gadget is going to give a few seconds of peace, and not much else.