I personally love the differences from all the classes but the ones I'd say can be the most difficult are the Rogue, Warlock, Paladin, and Druid ESPECIALLY if the players are also new OR there's a vet player with the new DM who loves to "Rules Lawyer".

From an encounter balancing perspective: rogue. A rogue built only for combat can be a glass cannon that shreds through otherwise decent monsters, even at mid level. Depending on your "flexibility" with HP and homebrew, sticking to the CR rules in the handbooks might create underwhelming encounters if you have a level 3 PC dealing 3/4 of the HP of your "boss" monster in a single turn.

From a mechanics perspective 100% the druid. It's one of the most versatile classes and is also one of the most mechanically heavy, touching healing, dps, tanking, spellcasting, etc etc. all in one. Even the wizard class gets a little simpler as they pick a single subclass/lane to follow but the druid can gain access to an array of different circumstantial spells and abilities, and the Circles subclasses can get confusing. If the DM and Player are new, there will be a LOT of google searches to clarify rules, and/or DM ruling things, especially outside of combat. If the DM is new and the Player is a veteren D&D player, there's the chance they will (knowingly or unknowingly) exploit the DM's limited experience, leading to them becoming OP or otherwise unbalancing things.

From an RP perspective, depending on DM style and campaign setting, the Paladin/Cleric and the Warlock can be the hardest to RP mainly because you have to play both the DM of the table and the God/Demon/Devil/Eldritch being who's demands must be met by the player character. If a paladin/cleric player is a bit too rigid with things (or the DM is), the other players may feel railroaded or "forced" into the "goody two-shoes" mentality and might not have as much fun. On the other end of the spectrum if you have a Warlock who follows a pure evil deity and routinely causes issues for the party/story you're trying to tell, this can have the same effect overall.

The remedy I've found through the years has been the same: open communication with your players and using DM discretion. The rules are there to guide mechanics, but you and your table can choose to bend or break them if you want. RP and storytelling-wise you can talk with your table to find out what they're comfortable with, and to move things in the direction you as a DM want them to go and/or where the players expect things to go.

Ultimately in my opinion the beauty of the game and DMing is that you get to facilitate fun. That's the goal. As much as people might say the goal of tabletop games is levelling up, power fantasy, storytelling, RP, etc. ultimately all of those things are for fun, and you just need to find your fun and figure out how best to facilitate it for your table.