I think pretty much anything is okay as long as you slice/peel it. I peel my lime/lemon, oranges, etc before putting them in. Obviously cut the apple/peaches off the core, etc. I keep the skins on foods like that though. Peanut butter is nice sometimes too but don't go overboard or else you might as well just put a straw in the jar of peanut butter lol.
When doing berries the seeds often don't break down but I don't mind that. If you don't want that you can just strain it before putting it in your glass/bottle. I would suggest limiting oranges to 1/smoothie unless you plan to strain or you really like pulp because it can be very pulpy. The first smoothie I ever made was 2 oranges, 1 lime, and 1 lemon. It was good, but I probably could have just used a spoon because it was so pulpy.
I usually just use what's on sale. I don't know what fruits are like where you are, but some get ridiculously expensive depending on the season. Clementines for example get stupid expensive here in the winter, but go into almost every smoothie in the summer because they get so cheap.