I haven't flipped or invested in any buy and holds YET! But working in the architecture field (high density residential to as low as high end single families), I deal with spec'ing and coordinating with manufacturers directly, all the time. BP had a podcast on similar topic about remodels for flips vs buy and holds. Flips purely depend on your comps. Buy and holds is about engineering the property with a balance of longest lasting while maintaining above market appeal. Understanding durability and maintenance of material is important, may cost a little more upfront but in the end it is worth it.
Thicker drywall (5/8s instead of 3/8)
Stain resistant paint or at least easy to clean up paint (this may vary cause in most causes tenants put so much holes and dents in walls, the turnover for rentals usually require repainting anyways)
Easy to clean back splash (less grout = less stuck dirt, so consider the size material)
Countertop type (granite and marble stains and require to be polished and resealed eventually; quartz is stronger and stain resistant, but cost twice as much)
For appliances, I suggest going to local appliance stores not big box stores. You'll get a better deal especially if buying in a package. Also will get better customer service and get quality answers to your questions. I always suggest for rental properties, get electric stove tops, never gas. Don't EVER let tenants deal with anything gas related, just so much liability. Get durable but affordable products. I know that's a contradicting statement, but don't go spending 10,000$ on appliances that you know tenants won't take care of. Balance it with reputable brands and quality product that will require less maintenance.
Floor tiles depends. If you want your property to have a good flow, you would ideally get same material as your living room, but putting wood floors in your kitchen is a terrible idea for rentals. At least go with the wood look long planked tiles that could imitate similar look while providing durability.
If you can fit a standard 10x10 or 12x12 kitchen layout, it will save you money as many smaller cabinet stores have deals for those. Home depot, Ikea, any big box store cabinets are cheap products, affordable, but cheap. You can honestly get real wood cabinets from small cabinet stores for comparable price.
Rentals = ENGINEER ENGINEER! Think durability.
Flips = depends on your comps, no reason to put 20k into a kitchen that will only raise the value a few thousand.