I'm thinking you might be expecting a single "best" answer, but a lot of these things are mutually exclusive. When I hear "hardcore game" I think of graphics and fantasy worlds and the things that make people compare an iOS game to a Playstation game...and those usually aren't the most addictive games.
For example, I think the game with the best multiplayer interaction is
Galcon. And it barely has any graphics at all. Complete other end of the spectrum.
And the most addictive? I hate to say this, but I've probably spent over $100 in real-world cash buying weapons upgrades in
Fieldrunners 2. Am I proud of that? No. Is that a sign it's addictive? Hell yeah.
I've also loved
Jetpack Joyride.
I wouldn't call a single one of those "hardcore," but they're the ones I've sunk the most hours into. (Outside of puzzle games like Bejeweled, that is, But I don't think you're asking about stuff like that.)
So I guess that answers half of your question?
This hits it right on the money. I'll list the games that are currently in my "Favorites" folder, but the "Hardcore" ones definitely aren't the most addictive. You really should check out Toucharcade.com, and their forums for more info on the latest iOS games.
The Walking Dead - If you play no other game, play this one! There's a reason this game got Game Of The Year from nearly every gaming publication (not just iOS or Mobile ones either).
Galcon - Great multiplayer game. This is the only game that has stayed on my phone, and received constant use, for years.
Heroes of Order and Chaos - This plays much better on the iPad, but still one of my favorites right now.
Fall Of Angels - Great storyline RPG
ShadowGun: DeadZone - Great multiplayer FPS
Modern Combat 4 - Great multiplayer FPS
GTA Vice City - No description needed
Infinity Blade 2 - Quick and intense fun if I have a few minutes
Uniwar - A turnbased and multiplayer online mix of Advance Wars and Starcraft
Ravensword 2 - The closest any mobile game has ever gotten to the Bethesda's Elder Scrolls series.
Anomaly KR - Fun take on a popular genre
Dark Meadow - Fun adventure game
Chrono Trigger - Still fun after all these years.
Real Racing 2 - Quick online racing game if I have a few minutes.
Nova 3 - Quick online FPS fun
Max Payne - Still fun after all these years
Orc - Mobile version of Diablo 3