I"m assuming this is for career mode, since the online servers were shut down long ago, although the LAN mode with a virtual private network could be used to play against other players on the internet (this was more popular for Underground 2, I'm not sure what it is/was like for Underground 1).
For career mode, the most difficult event is #107, 7 laps at Terminal in reverse direction, where the AI catch up mode is flawed and causes the AI to usually pass and/or stay in front of the players car unless due to luck the leading AI crashes near the end of the 7 laps. The AI are affected by the top speed of the players car, so as a strategy, switch to the Miata with stock transmission and no unique drivetrain part. This lowers the top speed of the Miata enough to reduce the performance of the AI cars. This strategy won't work with other cars like the RX7, because it has a high top speed even with a stock transmission. Before and after event #107, you can use other cars, but for event #107, the Miata is best due to being able to set it up with a lower top speed. Although the RX7 is the fastest overall car, the AI compensate based on top speed, so the RX7 is mostly best for setting best lap times as opposed to winning career mode events.
Given this, then the third unique part should be turbo/weight.
If you're going for lap times, look at youtube videos for Underground 1, since many them will state which unique parts were used in the videos. Also, with the original version of Underground 1, as on the cd-rom, the game ran faster on faster systems, the time clock would run faster than real time, and the car would be running faster, but displayed lap times would be about the same since the clock was running faster. A patch mostly fixed this, but with the patch, if the game starts to lag, the first thing it skips is events that tend to slow a car down, like landing from a jump, sliding on a wall, or even just cornering. The end result was players with slower systems ended up with faster lap times. Players with faster systems could slow the system down by recording a video while running an event, which would result in faster lap times. The video wouldn't be saved, it was only used to slow a system down, which resulted in better lap times with Underground 1, and some players used this back in the days of online play.
