Metroid. My first real exposure to the series was me playing the Prime Hunters demo on the display DS at Target whenever we went to the store and I had a lot of fun playing the demo. But the demo kind of mis-conveyed to me the type of game it was, I think, and I was like 10 and was more into the arcade-type appeal that they had set up for the game in the demo. When I finally decided to get the full version of the game, I was really overwhelmed and confused by how different it was from the display demo and had no idea how to cope with it, and had absolutely no idea what I was doing. I ended up returning the game for another 35 dollar game (which was Mario Kart DS btw and was my gateway to Mario Kart, all thanks to Prime Hunters).
I didn't really get to give Metroid another chance until I was older and more familiar with that type of gameplay, but honestly, I think if I played a 2D Metroid instead of Prime Hunters back then, I probably would've adjusted just fine, because it's would've been more familiar to the other kinds of games I played at that age.