Other motorheads may disagree, but these are my top 5 choices.

Toyota 4A-GE

Practically a dinosaur by today's standards. But this is the engine which powers the insanely popular AE86, or Hachi Roku as the Japanese prefers to call it. Weapon of choice by the Dori-Kin, Keiichi Tsuchiya. Pushing out 130 PS (97 kW) and 103 ft·lbf (140 nm), redlining at 7600rpm, this engine still manages to put some more modern engines to shame. Also the reason why you'll need to apply for a mortgage to buy a 25 year old Corolla.

Honda B16B

The engine which started the screaming revolution, an insane 9200rpm redline! Power tops out at 185 hp (134 kW) @ 8200 rpm & 110 ft·lbf (155 Nm) @ 7500 rpm. This is the metal heart which propels the psychotic Civic Type-R (EK9) to warp speeds, killing half of the Japanese teen population along the way. Naturally aspirated, substituting displacement with revs, the B16B easily humbles some engines twice it's size.

Mitsubishi 4G63

Bombproof, sledgehammer performance. The engine which powers the iconic Mitsubishi Evolution from it's first iteration all the way down to Evo9 (which pushes out 286 hp and torque to 289 lb·ft). The Brits went a step further with their FQ-400, squeezing 405.2 hp; and still backing it up with a factory warranty. Swapping the internals with forged components and adding a bigass turbo, the 4G63 was good for up to 4 digit horsepowers. This speaks volumes about the robustness engine. Rumoured to power the Nimitz class aircraft carriers when the nuclear reactors brokedown during Operation Iraqi Freedom.

Nissan RB26DETT

Overengineered monster created by the insane engineers at Nissan. The heart which powers the Godzilla; Nissan Skyline GTR R32, R33 and R34. So dominant was this engine+car combo in motorsport, it lead to the demise of of Group A Touring Car racing. It was like a DOTA game several years ago; when a noobish gay/mut/ikan combo tried going up against achew 3v1. It is often possible to produce 600hp without modification of the engine internals (by use of larger turbocharger, and either keeping the stock twin turbo arrangement, or using a single turbocharger). With extreme modification, the RB26 motor is capable of power in excess of 1 megawatt (or over 1,340 hp). Forget what the textbook tells you, this was the powerplant which helped bring Apollo 13 back from the brink of doom. 

Nissan VR38DETT

The spiritual successor to the RB26DETT. It's a little early to call this engine iconic, but for the new Nissan GT-R to lap the Nürburgring circuit in 7:29 sec; slaying the Porsche 911 Turbo along the way, it's just a matter of time before it achieves the same status. Handbuilt to perfection at Nissan's slaveship, this gem weighs in at 480 bhp and 430 lb·ft of torque stock standard. Figures like these mean the VR38DETT isn't an engine to scoff at. More German heroes will fall when the new GT-R V-Spec is released. The late Michael Schumacher was executed when he bought 3.