Golden Sun and TLA were both great games for their time, but Dark Dawn kinda butchered the series for me. The only really challenging fights in the original two are ones were the enemy has both multiple attacks and the ability to use Djinni Storm. Apart from that, it's not all that hard, although it is enjoyable. The puzzles were innovative for the times as well, using psynergy to affect things seems ingenious to me. However, Dark Dawn is quite different. You can mash A throughout every battle except maybe 3 in the game as you'll be fine. Enemies have less HP and give you more experience, the puzzles are easy and the dungeons are short, and the script is pretty lackluster.
GS and TLA: Good.
DD: Junk.