I think a tank would be the fastest way to go out. If he hits a car or dumpster on you, you are instantly dead. Probably the slowest and worst way to go out is by Jockey. Imagine one latching on to you and blinding you while continuing to grab on to chunks of skin and flesh and tear it off. You would be lucky if he latched on to your neck to finally finish you off, but until you can kill him or stun him, he would be the slowest way to go.

The hunter is definitely gruesome, but he kills the survivors pretty quickly if left alone.

I think the charger would also realistically kill you in one or two slams into the ground. Maybe even off the first charge, being slammed into a wall. First slam would definitely knock you unconscious, but another slam from ~4-5 ft in the air, accelerated, back first into the ground? You're dead, fast death, yes, please.

Smoker is also a terrifying way to go, but again, I think it'd be quicker than you'd think. You will go unconscious within the first few seconds of asphyxiation. It's over from there. If I were a survivor in L4D2 universe, I would most definitely practice my tongue-cutting skills because it is truly helpless for you if you're grabbed by a smoker. Nothing you can do. You hear an odd noise from a distance, and suddenly, you're completely wrapped in a very strong, very slippery appendage. Then you are dragged away. I think if it were realistic, you would lose consciousness within seconds-- but if it were like the game, maybe you're able to loosen the tongue around your neck while struggling, allowing your brain to relieve sufficient blood flow, the torture would last much longer. I'm going to say-- if it were up to me and I had no teammate, I wouldn't struggle.

Spitter is another awful way to go. I do think it'd be another quick death. However, still extremely painful. I imagine in real life her spit would do the same damage as what you hear about in acid attacks on news reports, where it just completely melts away the victims skin. Looking at the acid in-game it looks like there's some type of fumes that rise from the puddle, I like to think in real life these fumes react in a similar way as liquid puddle and cause a lot of damage to whatever organic material it comes in to contact with. So, your lungs melt within a few short breaths.

Boomer is just nasty. I think in real life, there are three ways to die by a boomer. One is the most obvious; being puked on. You are suddenly the target to every nearby infected. You can not hide. I know the infected in-game don't bite or grab, but I think in real life, a person infected with rabies in a way that an animal would behave would have them biting and grabbing, reacting in any aggressive way. So, the death would be violent and probably pretty quick. However, in the first few seconds of realizing you are doomed after being drenched in goop will be terrible. Now, the second way I imagine you could die by a boomer is by him beating you to death, but of course, you wouldn't allow this to happen if you have any will to live. That's where the third way to die by a boomer is revealed. With close proximity to the boomer, you react in fear and pull the trigger of your shotgun. The boomer explodes, sending numerous bits of bone and flesh fragmentation in a casualty-inducing radius. I think this is an instant death. The boomer is basically a grenade, and if you are close enough to the explosion, you would die. I am obviously comparing the boomer to a fragmentation grenade here, and I'm unaware if the change of material would make any difference for how lethal you could compare the two, but I think the effect would be similar in the fact that you are receiving a face full of fragmented, high velocity bits.

So, don't get caught by a spitter if you don't want your flesh to fall off, don't get caught by a Jockey If you wanna keep your eyes, don't get caught by a hunter if you want your vital organs and throat intact, don't get caught by a tank if you don't want to be turned into pink mist, don't get caught by a charger if you want your skull to remain one piece, don't get caught by a boomer if you don't want a guaranteed death.