Well, I wont vote cause I haven´t tested all of them but:
1) the best sim ever was GP2 (now its like abandanware, but I call it "the best ever" because if you mind that it was released in 1996, it was advanced for the time and I played almost till year 2000, very easy to modify)
Then GP3 came and it was a total crap.
Then GP4 was better, but still had the GP problem: front wing angle increase has no drag increase and car balance is very easy to reach modifying suspension (and diff ) and brake balance.
So, basically in GP4 you could run in a track like Indy with good grip and top speeds of 340KPH while computer rival cars tops 310KPH, than means 2 secs per lap.
Thats too boring. I know you can modify the game but the phisics engine is not too realistic cause of that.
NOTE: My old PC barely stands GP3, I played GP4 in a friends house and got bored in a couple of weeks
My next step is to buy a new PC (for some FEA programs I need) and take the chance to run rFactor or similar sims.
ADVICE: Ciro, dowload GP2 (only 50mb aprox.) car and track editor utility -nowdays is hard to find them, try here
http://www.grandprix2.de/ and track editor tutorial:
http://www.waa63.ch/racesim/TEIC/index.html
and dowload these tracks:
- bern-grauholz 1999

- Bogota, Colombia (I dont know it, just check it)
- Lake Joutjarvi - Finland (fictional track, some good curves)
- St Senoux - France (fictional track, good curves-countercurves combos)
- Cadwell Park - GB (also called mini-nurburgring)
- StuwLake - Netherlands (fictional track, a real challenge to drive)
- TunnelTrack - Netherlands (fictional track, a real challenge to drive too)
- Eagles Nest - USA (fictional track, look at last curve)
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna