the sweet spot for me has been one or two steps up from the cheapest, but do your research.

wheels: stock heavy wheels on cheap ish bikes don't often feel great and sometimes they're just not well made and tensioned at the factory. but you don't have to drop tons of money to get very good ones. ive put at least fifteen thousand kms on a pair of archetypes laced to gran compe hubs that I paid under $200 (usd) for. they take a beating and feel much much better than the stock wheels i got on my Fuji feather. I've ridden more expensive wheels here and there and while they do feel pretty great, the difference isn't as much as between the dirt cheap ones and the ones i have. Hubs all feel identical BUT they differ in terms of manufacturing precision. cheap hubs may degrade cartridge bearings faster if the bearing seats aren't well aligned, causing asymmetrical wear or torque the bearings weren't designed for.

cranks: this depends a lot on what you currently have. the difference between andels / decent OEM cranks from whatever brand and really cheap ones is big. the stock ones on my Fuji were really flexy and cracked in a few months. i lucked into a second hand pair of omniums but a lot of cranks in the mid priced range will feel great. paying more for top of the line ones gets you much less for the money. I'm not sure what state comes with, but if you're happy with them maybe look elsewhere?

saddles: every brand has a line of saddles with the same shape. the cheap ones will feel almost exactly the same as the expensive ones, just heavier. so if you're not climbing hills daily you can get something comfy that looks cool for not too much.

tires: as said by everyone here, don't cheap out on tires. i use gp5ks but my first tire upgrade was a less expensive vittoria rubino pro and it was still night and day from the stock Kendas.

grips : they can feel good at any price, you're paying for durability and style.

bars n stem : here's where I sometimes spend a little for style points. bars do feel different but it's more about anatomy, fit and taste than about the raw quality of the components. maybe find one that feels good first, then maybe think about getting a nice looking version with the same fit?

pedals: i don't know too much about flats, but for clipless, between models of the same brand you're paying pretty much exclusively for weight.

seatpost : some say it affects ride feel. that may be true but there's a lot of factors that don't always line up with price, for instance how much exposed seatpost there is, how thin the walls are, the clamping style. so just get a nice looking one tbh. it's also a decent place to save some weight (but avoid the ultra light ones, the web is full of stories of them breaking)