I don't quite agree about the Nook. I have a Nook Glowlight Plus, their newest ereader. They're still selling them. Except for a couple of big flaws they're really pretty nice. The big problem is the capacative home button which, if I touch it while reading, takes me back to the home page. I rarely get through a page without that happening. It's pretty irritating.
The other problem with the Nook is that it's dictionary is grayed out and I can't read it. Also their customer service is rather poor.
Other than those two problems it's every bit as good as a Kindle and Kobo. To me those are big problems so I don't use mine much. To a lot of people they're not bothered by those issues and they love the thing.
The Nook Glowlight Plus is also waterproof. I'm not sure how important that is. It's not something I care about but it's there if you want it.
My preferred ereader is a Kindle Voyage or a Paperwhite. I have both. I like the Voyage a bit more but it costs a lot more. Both are very solid and stable and everything works just the way you hope it will all the time. When you do have issues Amazon has great customer service.
I also have a Kobo Aura, one of their older ereaders, and I like it a lot. It's screen isn't as good as the Kindle or the Nook's screen but the newer Kobos have screens as good as the others. The Aura is older technology. That said, it's a decent screen. The newer ones have excellent screens.
The Kobo store isn't as good but it's okay. My two experiences with Kobo customer service have been pretty bad. They're nearly impossible to get in touch with. Once I did (it took nearly two weeks of trying every day) they were helpful.
Considered simply as reading devices I think Kindle and Kobo are the better choices. Kindle is more stable. Everything just works. Kobo has more ability to control fonts, etc. and they work a lot of the time. They fail to work a lot too. I buy my books from Amazon and convert them and side-load them to the Kobo and I use whatever font features happen to work. But reading on it is always still a good experience. The features that sometimes fail on the Kobo don't exist on the other readers so not much is lost.
There's a freeware program called Calibre that's excellent for converting between formats. It's trivial to use and extremely quick and very reliable. I can load it, drag a book into it and convert it to the other format and have it ready to go in less than a minute. It also does bulk conversions so doing a bunch at a time is about as easy.
Books converted between formats are usually as good in either format. Sometimes a table of contents won't convert correctly because of a problem with the way it's formatted. Calibre also supplies tools to fix that. But 99.9% of the books convert with no problem at all.
Calibre also lets you install your own fonts if you aren't happy with the fonts in the various readers. I use this occasionally for some special reason but I'm fine with the readers fonts as a rule.
I suggest a Kindle or a Kobo. Make sure you get one with a light. Which model doesn't matter that much. They're all good. There aren't any to avoid. None of them have problems. Chances are whichever you choose will be fine and you'll feel like you made the right choice. Especially if it's a Kindle Voyage. I'm entitled to a little prejudice, I think.
Barry