Oh man...this takes me back.
First game I truly adored...probably Starwing. I liked the original Super Mario Bros but it was Starwing that hooked me for life. I still play it every now and then. My SNES is still set up not five feet from me and this is still inside. Close second would be FF7 but Starwing beat it by a few years.
First game I can honestly say I detested beyond all measure...Sonic 2006. There were games I didn't like long before this came around, like FF12, but this was the game that made me cry. Defacing my childhood icon like that. Everything about that game felt like I was being sexually assaulted by a very angry bear. The graphics sucked. The level design sucked. The music (bar the final boss theme) sucked. The creepy Elise/Sonic romance should be considered a crime against not only humanity but against every species to ever exist anywhere in the universe at any point in time. It made me sick.
A close second would be FF13 for showing that nobody lived forever. Even the mightiest of gaming legends has to fall sometime. And it seems the Final Fantasy series is joining Sonic on life support. Only now it's worse for Final Fantasy 'cause the twitching corpse of Sonic is about to spring back to life with Sonic Generations, which if the demo is anything to go by is gonna kick so much ass we forgive them for Sonic '06. And Sonic Shuffle. And pretty much everything since Adventure 2. Which would leave Final Fantasy as the poster child for falling from grace and landing face first in a pile of dog shit.