>>55665923>Red, BlueIt's pretty archaic but that might give you enough interest to keep playing since it's so far removed and different.
>Gold, SilverIt's quite beautiful but there are a few places you might feel lost and lose motivation to continue, once you get. It depends on your definition of beating a game because most people finish the post game in GS and consider that the true end.
>RubyThe front end is smooth but a lot of people give up after they hit the water, it's not for everyone.
>DiamondAll the TMs and samey map makes a lot of people lose interest.
>BlackThere is a lot of dialog to start but once you start the game it's pretty easy and all the new Pokemon keep it fresh, there are some parts of the map that get tedious and make you say "fuck this".
>XThe characters are verbose and annoying but it's pretty easy game with a big selection of Pokemon. There are parts like in Lumiose city where you might get lost running after Lysandre to continue the story.
Out of all the games you have here is how I think they rank for an ADHD child to complete, easiest first:
X>Ruby>Black>RB>GS>Diamond>GS-postgame
TLDR: Don't feel bad about not finishing a Pokemon game. It's more common than you think. It's a portable franchise, you pull it out when you have some time, fuck around, and put it away. In every game there are parts that suck to play for long periods of time and there are a lot of things to do other than completing the game. Have your own fun.