I haven't played most of these games in ages, but going from what I remember:

Dragon Quest 2 - Getting to Hargon's Castle.

Dragon Quest 4 - For as much as I love the characters in the game, and the idea behind Saro, the game, mechanically, doesn't really do anything for me. Not being able to control your characters is extremely annoying, and the character development freedom of the previous game is gone. Kind of like an FF3 vs. 4 thing.

Dragon Quest 6 - This is a weird game for me because, while I don't remember disliking anything about it, I always stop playing it for one reason or another part way through. Mechanically there isn't anything I dislike about it, but it just doesn't do anything to hold my attention.

Dragon Quest 8 - See above, only I really like the cast. I have a save file hanging around somewhere near the end of the game, so I'll get around to finishing it some day. Probably. Maybe. If I don't just start over.

Dragon Quest 1 - Basically what Fynn said -- the game is a simple, no frills RPG with some interesting little bits (rumor has it you might not have to save the princess . . . ). It and the original FF are my go tos for quick little RPGs, with FF1 being for my more ambitious moments.

Dragon Quest 9 - I . . . didn't initially get behind this game, but once I hit the Dharma shrine it rapidly grew on me -- I am a complete sucker for DQ3 style character creation, and the story wasn't too terrible. What really ended up getting me were the bonus dungeons though, especially once I found out the final bosses from previous games popped up, and I spent waaaaaaaaaaay to much time with that stuff.

Dragon Quest 3 - DQ3 is basically my favorite RPG on the NES, and easily my favorite in the series. I love exploring the world for all the orbs, I love effectively creating my own classes, I outright adored the second world and ending reveal. <3