What's the Ultimate Infiltrator?

[Wasp]

What would you say is the Ultimate Infiltrator in 5e? In the sense of a character that could infiltrate an organization or a court or something similar and stay there undercover for a longer period of time gathering information, stealing stuff or manipulating the people without suspicion maybe even impersonating a specific person?

Is it an Assassin Rogue? A Mastermind? A Changeling? A Warlock with the Mask of Many Faces? A Lore Bard? A Sorcerer with Subtle Spell? A Knowledge Cleric? A combination?

[Dudewithknives]

A moon Druid

[NaughtyTiger]

eberron changling... any class

[8wGremlin]

Eberron changeling moon druid

[Treantmonk]

Variant Human Warlock (maybe Fey pact) with Actor feat and Mask of Many Faces and Misty Visions with the Charlatan background. This becomes an amazing infiltrator at level 2.

At level 6, the Whisper Bard becomes maybe even better.

Neither require any platest material, the CHA focus aids in interaction, and both will be capable in combat as well (if built well)

[Lunali]

If you're going for long term infiltration, disguise self is not reliable. Even if you can consistently avoid being touched, at some point you're going to have to interact with people for more than an hour.

Since the question is about the ultimate one, level shouldn't be a question, so a changeling mastermind would be my pick. Reliable talent is very useful for long term action as small mistakes tend to snowball over time. Combining a flawless disguise with the ability to lie to Detect Thoughts makes for an impressive cover. If you feel threatened by shapeshifter specific detection, you could always wear a disguise as well.

[JakOfAllTirades]

I'd go with the Bard College of Whispers.

[Unoriginal]

Anyone with the Actor feat?

[carrdrivesyou]

Be a changeling mystic. Take the actor feat. You will be practically undetectable and you will be able to do all kinds of off the wall stuff. Include read minds. Go nuts.

[Corran]

This is a very open ended question and you can go in many ways about it. According to what exactly you were planning to do, some options lose and others gain value.

A level 17 mastermind rogue with expertise in deception and persuasion (and the diplomat feat if UA is allowed) though is a simple (assuming you play at high levels) but effective way to go about it.

[LudicSavant]

Druids are actually really good at this sort of thing. A couple people already namedropped the class, but here's a few of the reasons why they're good infiltrators:

Pass Without Trace? Check.
Can be a literal fly on the wall? Check.
Can use little critters as spies and informants? Check.
Magical mobility options for second story work? Check.
Can gather information via divinations? Check.
Is very perceptive and insightful? Check.
Is so SAD that aside from Wisdom they can invest in basically any stats they want for skill check purposes? Check.
Has excellent tools to boost all of their ability checks? Check.

Druids are magical masters of disguise, hidden in plain sight, keeping tabs on civilization from the shadows... manipulating its leadership to nature's ends. Why do you think there's never been an industrial revolution despite the age of elven civilization, huh? Yeah. Medieval stasis is engineered, my friend.

How come you didn't know that that was what most Druids do? Because they're that good at being secretive.

[kamap]

A changeling as race to start.
If its just to snoop around and gather inteligence a dash of druid, change yourself in a fly and literally be the fly on the wall.
Become the boss' cat and be in all the important meetings.
A simple bird, rat or snake that can get in and hide, i could keep on going.
To impersonate people a dash of warlock to get the mask of many faces invocation.
Coupled with the actor feat and disguise kit will get you far.

[LudicSavant]

Unbeknownst to the king's court, everything said in the vicinity of the courtyard gardens or beautiful hanging vines throughout the noble villas... is overheard by the plants, which report to the Druid spymaster, who has been wearing the face of the countess for the last 3 years.

[Ivor_The_Mad]

Changling Assassin 13/Moon Druid 7 with the actor feat and proficiency in persuasion, deception, performance, stealth. Max Cha and Dex. Some magic items couldn't hurt either. Also, a pet such as a Tressym would help (see invisibility and can fly not to mention the great dex/stealth and perception) these things make great scouts.

[tieren]

You'd need at least 8 levels of druid to be a fly on the way (can be an ant with 2).

What level are you building for? Recommendations above are good in general. If you want a level 20 build I would definitely want some sorcerer for subtle spell, some druid for the reasons above, and some whisper bard (expertise, and jack of all trades, etc...)

Raises an interesting question for me. If you have subtle spell and therefore don't need verbal or somatic spell components, would you be able to cast in beast form? I thought the main limitations were related to the physical restrictions of the animal forms (being unable to speak, having wings instead of arms, etc...)

[unusualsuspect]

Changeling for at-will disguises, skills, stats, and expertise in a tool.

Arcane Trickster Rogue 3+, for some pretty impressive object interaction so long as you have Mage Hand up, and a few other Cantrips as needed beyond the usual goodies (particularly skill-wise), and sneak attack, with potential for a lot of useful defensive bonuses and perhaps surprising mobility. An extra ASI is always appreciated, to get Dex and Cha to max and still take Actor as a feat, if you make it to 10+.

It should surprise no one that Assassins make excellent assassins, though the non-assassination benefits of Arcane Trickster really makes me lean towards AT even over Thief (with Mage Hand up, you can do basically everything Thief can do as a bonus action, but at range).

Gloom Ranger 5 is enough to give you three 2nd lvl ranger spells known (Rope Trick and all it's useful "rest for the infiltrator" potential, plus some mixture of Healing Spirit, Pass Without Trace, and Silence all look worth while) and Extra Attack to make sure at least one attack gets sneak attack off, plus all the lovely benefits an infiltrator could possibly make out of being invisible to people's darkvision in darkness...

Warlock 3+ gives a few more useful tools in cantrips (including the best combat cantrip in the game), short rest spell slots (combine's really well with Rope Trick for a quick Short Rest, but also includes things like Invisibility and Darkness) and Invocations (Lots to choose from that can help an infiltrator, and if you can spare one for Agonizing Blast, you have a very viable combat tool in Eldritch Blast). Pact of the Chain can give you an intelligent, invisible-at-will familiar - an extraordinary tool when combined with a Legerdemain'd Mage Hand for ranged, subtle manipulations and espionage. If you plan to go to 9+ Warlock, going for the Book of Ancient Secrets invocation can yield some great divination rituals (and the lower level rituals aren't bad either - with that invocation, you can take any ritual spell, not limited by class.

The Actor feat seems like a no-brainer, but Keen Mind also seems exceptionally useful for an infiltrator trying to overhear conversations.

Disguise Self isn't a bad thing to have access to as well (which you do, through Gloom Stalker), since it can change the appearance of clothing in ways the racial ability can't. Given that it can combine with the racial ability to frame people in various ways (and given you're equipped through magic and/or guile to escape just about any situation and leave only someone's good name to take the punishment), it actually remains a powerful tool.

Lots of magic items can also supplement things, but aren't really necessary on a day-to-day basis. Aside from your thieves' tools personality, use throwaway identities and magic to sneak, talk, and/or magic your way through any barrier between you and the object of your quest... whether that's secret information or an enemy's life, you've got amazing tools to get access to it.

[strangebloke]

Druid is the best, but for long term cons there's also the warlock.

High charisma, at will disguise self, at will invisibility, at will silence, and a kick-ass familiar with greater invisibility.

Plus, with a slow adventure pace, the warlock can take short rests all the time, so he never runs out of juice. Toss in a couple levels of sorcerer for subtle and you're golden.

Archfey and GOO are my picks for subclass.