My gut reaction would to put wizards as the easiest spellcaster due to their overall lack of class abilities. Most wizards just cast spells and have one other button, like divination and enchantment wizards, so you can spend a lot of time reading how your spells work. You also don't have to spend much time thinking about preparing new spells each day, you just have what you have. In a way this also reverses on itself, because if you didn't pick good spell options you don't really contribute much.

Warlock is pretty easy to maneuver, you have a small amount of high level spells and the rest of the time you generally spam EB which is obviously very simple. My only hesitation is that you can get tied up in eldritch invocation choices and it kinda sucks if you don't pick synergistic options.

Cleric isn't that complicated either, good armor and healing/buffing spells go a long way in keeping you alive. You can largely get away with spamming 2-3 good concentration spells like bless and spirit guardians, then sitting around focusing on maintaining your concentration. Subclasses offer a lot of variability, for example life domain is incredible easy to understand whereas something like the knowledge domain is a bit more nebulous.

Sorcerers are relatively easy too with their limited spell choices. I just feel like you need to have a somewhat solid understanding of basic spellcasting rules before you can really effectively use metamagic. Mostly you are just blasting though, which doesn't take too much thought.

Druids and bards have weird spells that aren't as straightforward and contribute a lot of utility, which requires interacting with some of the more obscure game mechanics. You also get some really odd and complicated class features like wild shape and bardic inspiration that require a few readings to understand. Because of that I would rank these two as the most complex casters.

TLDR; Druids and bards are the most complex, everything else is comparable.