My backlog is plenty long enough so I'm all for skipping games that aren't worth it.
What KH games should are worth playing? I imagine KH1 and 2 would be, but of the DS/PSP/3DS games, are any worth it?
The "mainline titles" as in the ones that are not filler and that have brand new original content are 1, 2 BBS and DDD. In addition to those Chain of Memories is the most important storyvise but the battle system with cards is a major turnoff to some. Here would be my foolproof guide to a newcomer to the series:
Step 1
lay KH Final Mix on the 1.5 HD collection. If you absolutely hate it the series is not for you. If you feel like you enjoy everything else about it but the battle system and camera controls and general early ps2 era clunckyness you might enjoy kh2 more so hang in there.
Step 2: Try Re:Chain of Memories on 1.5 hd remix. You should know very early if you hate it, can endure it or even like it. If you hate it ask yourself if you are invested in the story enough to watch the main story cutscenes on youtube. If you hate the game, dont care about the story and liked KH1 and just want a true sequel to that skip CoM and go straight to kh2.
Step 3: Wait for 2.5 to release. The KH2 included there is much improved and has a ton of extra content compared to western ps2 release of kh2.
Step 4: Play KH2 on 2.5
Step 5: You hate the story and characters? Skip 358/2. You cant get enough of the story? Watch the 358/2 days "movie" on 1.5.
Step 6: Play BBS on 2.5
Joke Step: Cant get enough Nomura in your body? Watch the Coded cutscenes on 2.5. Hate the story but like the games? Skip Coded.
Step 7: play DDD