Bioshock is better than halflife, But..

You're comparing games that were released in two different gens.I know halflife 2 was only released for the x360 and not for the xbox, but it's not realy a game of this gen. More like something in between.

You could have easily maxed hl2 out on a single core cpu, and a geforce 4. The consoles of this gen are much stronger than that. But the old xbox had a geforce 3 (which is nearly the same as a geforce 4), the only thing that was not enough was the cpu (700 mhz), Well maybe just barely to run on the lowest settings.

One of the biggest selling points of halflife 2 was it's havoc engine, that could provide us with decent physics. Fear and far cry, which were crossover-gen games, had also a nice physics engine. Halflife was still the best at this physics but also the most taxing on the cpu. Far cry is the only game that was still released for the old xbox (but not the ps2, because the ps2 couldn't handle that) because far cry didn't use as much physics, and the graphics engine was very scalable.

Fear was the most taxing of the three, especially because of the graphics, and was therefore also passed on to the x360. But it ran fine on single core cpu's, and could actually run (allthough on the lowest settings but it still looked good) on a geforce 3.

Bioshock however is a whole different animal. A single core could officially run it , but it was a bad idea, A dual core was highly recommended (something halflife didn't even use) and the minimum gpu requirements were more than what halflife 2 needed to be maxed out ( a geforce 4 for hl 2, and you needed a geforce 6600 at least to play bioshock, and that was way to optimistic)

So off course bioshock to me is way better, but halflife 2 was groundbreaking because of the psychics engine and this gen it's just normal. Imagine you had the physics of previous gen? there were no physics lol. Allthough there was one game that actually used it a little and that was hitman (released in 2000) They used only physics on characters not on objects, the so called ragdoll physics , something that today is also considered normal.That's something a lot people don't realize, how groundbreaking hitman was.

But i'm deviating to much here (this is fun stuff to talk about though). The thing is you can't ask me to say what game's best,

It's like comparing the first game that used 3d accelaration in a perfect way (quake), to  AAA+ games where this was already standard, and there were able to add a lot more stuff in the story, artwork, gameplay or even newer technologies to make the game more realistic. Like halflife, unreal, deus ex , medal of honor or even the first call of duty, and ... delta force.