Half-Life 2 has the most powerful and realistic feeling explosions of any game ever.
There's no showy Michael Bay style ball of flame moving in slow-motion but instead an extremely quick bang. Understated but powerful and very effective.
When you're in an enclosed area and a stray bullet from a Combine soldier strikes an explosive barrel, lighting it up like a fuse, there's little that you can do apart from back away as much as possible (which is often not very far), crouch down, squint your eyes and hope the blast leaves you with at least some health points left. BANG! There's a quick flash and pieces of debris are violent blasted at your face at a thousand miles an hour. Your ears are ringing, you're alive although only just. "Vital signs critical" claims your HEV suit.
This.
so many of the current games have such similar looking explosions, all way too movie/pyrotechnic styled. Loved the dense feeling of the HL2 explosion, very different, some how more believable to me.