"most realistic racing game"
Having the thread title in mind, I'll have to think about what, in my opinion, makes a "realistic racing game". That'll be the following:
- Graphics
- Sound
- Physics
- AI
- Grids
- Tracks
Then, I'll say what are my all time favourite PS2 driving/racing games:
- GT (3, Concept and 4)
- Enthusia
- Toca Race Driver 3
- Le Mans 24 Hours
GT games win in the graphics department. They also win in the "size" department, but that doesn't make any game "realistic", it just makes it big. Both sound and physics are good, but not great. The AI is horrible, the grids are of 6 cars and not always evenly matched, the tracks are very good, but real life tracks are just a a few (even if GT4 has two of the best tracks in the world, the Nurb and Le Mans).
Overall, it's a very good game. However, the lack of proper AI and the "6-car-grids" don't make it a realistic racing game.
Enthusia has good but not great graphics and sound. It wins in the physics and AI departments. The grids are more evenly matched than in GT, so the races are more interesting in this game, but it's only 6-car grids again.
It's a smaller game, with a lot less cars and tracks than GT, and some of these tracks are a bit ridiculous (going through caves is a good example of what shouldn't be in this game

). Thi game also features the Nurburgring, but it is too smooth, when compared to GT4's recreation. And it has some very good fictional tracks where it is a pleasure to drive and race.
Overall, Enthusia is a more
realistic racing game than GT, because of its physics and AI
Toca Race Driver 3 is very different from GT and Enthusia. Its physics are more arcadey, its graphics and sound are just not at the same level. But ... the AI really fights you (sometimes a bit too much) you get up to 20 car grids, and loads of real life awesome tracks, all missing from GT and EPR, from Bathurst to Spa, from Phillip Island to Indianapolis, from Brno to Hockenheim, and so on ... ( did I say Zandvoort and Silverstone?

).
And, it's the only of these games where you can play
online, with online features unrivaled so far, even in the PS3. So, you get all that against human players (or a mix of humans and AI to fill in the blanks in the grid, as you wish).
I have raced many, many hours/nights, week after week, month after month, of TRD3 online, made a few friends there and I have to say that no other PS2 game could keep me busy racing for such a long period.
So, TRD3, as far as "realistic racing" is concerned, is also a good title contender, inspite the poor physics, graphics and sound. It wins over GT and Enthusia, by a long shot, in the tracks department and the grids size, and it's better than GT in the AI department (not difficult), maybe on par with EPR here. Being the only of these games that is playable online gives TRD3 an enormous advantage, but only if you want to play online, of course.
Le Mans 24 Hours is the older of these games, the graphics, sound and physics of it are more apropriate for a PS1 generation game, but I stick it here with the other three because I still think that my most realistic racing experience comes from this game. Of course I am an endurance racing fan ...atic

but if you want to do the full 24 hours of Le Mans in a game, it's this one you have to pick. The grid is still smaller than in real life (24 cars, when the actual race has about the double of that), but you have night and day, very well recreated pits and pitwork (better than GT4, and that is amazing), random weather, and the cars you race against don't have any "arcadey" behaviour (no rubber band effect, no rabbits), they just do the full race at the pace they're supposed to do, and either you can keep up (with the Audis and works Panoz and Pescarolo), or you can't.
All in all, if you want to buy just one game, it's a tough call. If I was in that position, I would probably buy Enthusia. But I would miss the other three, because they all gave me a good gaming experience.