Not a biggest expert on graphical designing but in usage terms, both Adobe Photoshop and Paint.NET are good for making decent quality images that may cover your needs as it is used by most of people.
No no no no no!
There's tons of editors out there and you can't say there is one that is "best". Each person has his own tastes. You'll just have to use a few editors for a couple days or a week each to see what you like best. When I first picked up Photoshop I really disliked it. Sure, it has tons of features but do you really need all of them to create a CS2D sprite? No! Pick what you're comfortable with rather than what is considered most popular. It's common sense!
I personally settled with PhotoShop Elements 5.0 since it has keymapping for my tastes and is fairly easy to understand. Before that I tried Paint.NET, Paint Tool SAI and they're also pretty good editors but Paint.NET had this weird thing with alpha layers and different keymapping while PTS interface just didn't grow on me. Also PSE 5.0 has tons of crap I don't use or understand but ye, it's still worth a try...
TL;DR try all kinds of editors and see what you like best!
Well, as Rainoth said, there are no "best" photo editor.
Each has a flaw, like Rainoth said about PDN. PS for you, a newbie, will have to read the manual on how to use it or otherwise, you won't get used to many key binds, so does Paint.SAI.
PDN, in my own opinion, is the best for starters.
In just snooping around tabs and checking the functions, you can familiarize yourself in a short time, thanks to it's easy to use interface. Since i'm a lazy bum and pretty much hyped to learn pixel art in my noob days, I tried PDN, and now i'm using it for three years now
Anyways, PDN, for me, is the gateway to learn how to use PS too, so i'd say experience is the best teacher.