I think 4 is the best. The engine is very well isolated and exposed and composed. Crisp, clean, professional, and intentional looking.
The other photos all suffer to varying degrees from distracting elements and confusing composition. For example, #5 is an attractive and interesting photo. The HDR (I think?) is nicely done without going really overboard, the dome plays off of the lines of the car interestingly, and the guy in the red shirt is intriguing. However, I really doubt that your intention was to highlight the guy in the red shirt or the dome. This is a car show, and I'm guessing your goal was to show off cars as your main subject. In this photo, the man in the red shirt, however, seems obvious to me as the clear focal point of the photo. Red on green pops out, he is isolated, he has a mysterious air about him, and the frame is composed in such a way that it looks designed to include him prominently. My eye darts to him more than anything, and thus I don't really pay much attention to the car. The car is not so interesting in this photo by comparison. Still a great photo! I'm just not sure if it's an intentionally great photo.
The remaining images also have distractions, but not in a potentially good way like #5. Just in a plain distracting way. The way-too-in-focus house in the last image draws my eye as much or more than the car; the bright magenta tape, curb, and decapitated man in the background aggressively draw my eye away from the car in #3, and the telephone lines in #1 and #2 have much more contrast than the car and draw my eye in those photos. Also the blue street sign a little bit. It all ends up having the same sort of feeling to it as if you invited somebody over to dinner but didn't pick up the laundry lying on the floor. To look super high quality and professional, you need to remove all the distractions from your subject and "clean house."
The good news is that all of this is a very consistent trend, and it's all the same problem, making it easy to fix it all in one fell swoop by doing some simple things:
1) Use much wider apertures and longer focal lengths to blur out your backgrounds much much more than in these images. If the house and telephone wires and so forth were part of an f/2.8 image at 150mm, for instance, they would be so blurry as to not distract at all, even if in the shot. Blurry backgroudns also just look nicer and more professional, when your subject is one specific object in the foreground like a car.
2) Pay a lot more attention to distractors in your images. Bright colors, contrasty things, in-focus faces and people, strong lines all draw interest away. Your subject (the car) should ideally be the highest contrast thing in the whole frame, the most in-focus thing in the whole frame, AND the most brightly colored thing (if color is a defining feature of the car) in the scene. This will require putting more effort into composition in a number of ways:
2a) Waiting for people or other moving things to disperse or arrange themselves into favorable positions.
2b) If possible, physically walking over and moving things out of the way like bright orange traffic cones, etc.
2c) Trying out more angles to see if distractors can be hidden behind stuff or positioned out of frame so as not to distract aymore
2d) Using blur and depth of field to remove any remaining distractors that simply can't be avoided
They're really really expensive, but a good tilt shift lens would also really help with pro level car photography. Usually cars are shot at an angle, which would make it difficult to use wide, blurring apertures without blurring part of the car itself. If you can tilt the plane of focus to line up with the car, however, then you can use a very wide aperture, blur everything BUT the car, and yet still have the entire car itself nicely sharp.
Just something to keep in mind if you're super serious about car photography in general (I don't know if this is a long term hobby or just an event you went to randomly)