Ubisoft has been constantly adding to the ever-evolving roster of operators in Rainbow Six: Siege since launch, and they don't appear to be stopping anytime soon. There are currently forty-eight different operators to choose from; twenty-four each on Attack and Defense. Each of these operators has their own unique gadget (and, sometimes, weaponry), lending each of them to their own unique playstyle.
Some operators are going to end up being useful more often than others, and this list is going to go over ten of the operators on Attack that are going to be useful more often than not. They aren't listed in any particular order, but each operator here is going to come in handy on just about every objective site in the game.
10 Thermite
Just about every objective site in the game has walls/ceilings that are going to be reinforced by Defenders. Breaching open these walls/ceilings is likely going to cause the enemy team a headache, and there's arguably no better operator at this than Thermite. Thermite is a 2-speed 2-armor operator, coming with two Exothermic Charges that can blow open reinforced (or un-reinforced) walls and ceilings, provided they aren't being Defender by a Bandit, Kaid, or Mute. Opening these walls makes the Defending anchor(s) job a thousand times more difficult, and can really make a big difference in the round.
Aside from that, Thermite has a decent primary weapon.
9 Hibana
Hibana is the other true hard-breacher in the game. She's a 3-speed 1-armor operator, coming with three shots of her X-KAIROS Pellets that burn through reinforced walls or ceilings that, again, aren't being defended by the Defenders listed above. These X-KAIROS Pellets don't open holes nearly as big as Thermite's Exothermic Charge, but they can be used from a distance, keeping Hibana a bit safer while using her gadget.
Hibana has somewhat-decent weaponry that would be a lot more effective with a larger magazine size. Despite that, she does have a secondary sub-machine gun, alleviating that pain a bit.
8 Thatcher
Thatcher is the perfect companion to a hard-breacher. Thatcher is a 2-2 operator, and his EMP Grenades either disrupt or outright destroy any electronic Defender gadget in the vicinity. This allows him to completely negate the efforts of operators like Bandit, for example, allowing the Attacking hard-breachers to do what they do best. (Unless the Defending Bandit is willing to put in some extra effort) It also makes him a prime counter for operators like Valkyrie, Lesion, and the sort, allowing him to destroy their gadgets without even having to see exactly where they are.
Thatcher also has a couple of solid primary weapon choices.
7 Maverick
Maverick is the only other almost-hard-breacher in the game. He's a 3-speed 1-armor Attacker, and his Blowtorch can open up holes in reinforced walls. These holes are slow to open areas wide enough to crawl through, but is definitely useful in opening up small sightlines that may catch Defenders by surprise. The Blowtorch can also destroy Maestro's Evil Eye cameras, or be used to enter through the bottom of barricaded doorways almost silently.
Maverick also has an exceptional primary weapon.
6 Buck
Buck is one of the most versatile Attackers in the game, making him useful in just about every single objective. He's a 2-2 Attacker, coming equipped with an under-barrel shotgun called the Skeleton Key.
This shotgun is fantastic for creating some vertical play by opening sightlines above or below you, but it's also great for quickly breaching through a wall, or just in combat. This allows Buck to have both a shotgun and an AR or a DMR, along with the frag grenades he has the option to bring.
5 Sledge
Sledge is another operator great at breaching. He's another 2-2 operator, coming equipped with a Sledgehammer that can open any soft wall or ceiling in the game. He can open up a huge area above objective in some sites, making like exceeding difficult for Defending anchors. He also has frag grenades he can bring to the fight.
Along with all of that, Sledge has some decent weaponry, including a secondary sub-machinegun.
4 Zofia
Zofia in another incredibly versatile Attacker. She's a 2-2 operator, and her gadget allows her to fire both two concussion grenades and two impact grenades. This gives her excellent breaching capabilities coupled with some serious fragging potential. The concussion grenades will detonate immediately when near a Defender, so they can be used to gain some intel into a room that you're unsure about.
Zofia also has some pretty decent weaponry, including the same pistol with a reflex sight that Ela has.
3 Ash
Ash is generally considered the best entry-fragger in all of Rainbow Six Siege. She's a 3-speed 1-armor operator, with two Breaching Rounds she can fire to open soft walls or ceilings from a distance. (These can also destroy a plethora of Defender gadgets.) Her speed and ability to quickly open up entry points makes her a force of nature that's especially intimidating in the first thirty-seconds of a round when the Defenders may not be one-hundred perfect ready for her just yet.
Ash also has one of the best weapons in the game in the R4-C, which is still fantastic despite the recent loss of it's ACOG.
2 Montagne
Montagne is one of the most disheartening Attackers to find yourself up against. He's a 3-armor 1-speed Attacker, coming equipped with a shield that can extend to protect his body from a full 180-degrees. This allows him to gain an unprecedented amount of intel without putting himself in much danger. A well-coordinated team with a Montagne is going to ensure Monty is flanked by at least one other person, giving them moving cover to use. As long as someone else is watching Monty's flank, he can be a real force of nature.
Because of his shield, Monty can only be equipped with a pistol.
1 IQ
The perfect counter to the current trap-meta in the game right now, IQ can see any Defender gadgets that run on electricity through walls/ceilings, provided they're in range for her. She's a 3-speed 1-armor Attacker, and her ability to gain intel on the Defender's setup is exactly what's needed when most Defenders nowadays rely on so many traps. Other than Welcome Mats, IQ can easily spot most enemy gadgets, and if the gadget is behind a soft wall/ceiling, she can destroy them without putting herself in harm's way.
IQ also has some great primary weapon options, making her a great choice in a lot of different situations.