E natural.  All knowing ears will be cocked to know whether you play correctly e natural or the erroneous e-flat. (Paraphrased from Ernest Hutcheson)

Good luck.

 Dear Orlando:
 It's a C-minor chord. There no possible doubt about it. Analyze the harmonic progression you have each bar, and the whole harmonic structure of the first four bars and you will notice that. In measure 3, the chord on the second beat is a C7, i.e., a secondary dominant to the subdominant degree (which follows on third beat). So, there is no connection between the function of the C chords of the second and fourth beat: one is dominant, the other is tonic.
 It becomes more clear if you make a schenkerian analysis, and notice that basically you have in the first four measures a simple progression of i - VI (- i) - V: the other chords just extend this.
 Furthermore, you have an echo of this in the second group of bars (5-8), where in the same metric point you have a c-minor chord.
 Best wishes!
 P.S.: By the way, the Urtext shows a flat on this particular e.
 

 (EDIT: can't stop posting...you already wrote what I would)
 Dear Orlando:
 Just for the sake of curiosity, I did listen to all my recordings of this preludes, to refresh my ears. Interesting results:
 - E natural: Rubinstein, Cortot, Pollini, Perlemuter;
 - E flat: Argerich, Lugansky, Zaritzkaya, Biret, Ashkenazy, and the orchestral version made by the composer Jean Françaix.
 So... Argerich versus Pollini ... Rubinstein versus Ashkenazy ...  
 What other versions do you know?
 Best wishes!
 

This is quite interesting. Ive heard both versions now, and the e natural sounds very wrong to me in this type of progression. The melodic line gets totally messed up:

g - ab - g - (f - eb)
eb - f - eb - (d - c)
d - e - g - (f - e vs eb)
d - g - h - (a - g)

As you can see, all the phrases ends with a whole step, so if the f would go to e natural, it would mess up this system.

Btw what follows this chord is surely not an a-flat, but a D7 chord.

We have an a flat octave followed by an a flat chord in the RH with an e flat playd with the thumb, so possibly a flat second inversion, anyway the chord that follows this is D flat. So we have C minor - Aflat major - Dflat major. Unless you have a good explenation my blood pressure is going to go up.

For the record, the chords discussed is in the 3rd-4th measure, 4th-1st beat.

The chord you are describing is the 1st beat of the 2nd measure. Get your facts right.

 WAW, just heard the Cortot interpretation, and he plays C major, F**ck . Looking at the score we have a C mjor chord indeed. I think i will go and jump of a tall building.

in busoni's variations on this prelide, there is an e natural written everywhere that part shows up.