I pick Lotus II, simply because that's the one I've played the most with my cousin (maybe completed the game - with cheat - around 100-200 times).
But the first one is very special too, the sounds were better, speed was faster, the crashes were more realistic (in the sense that you lost much more speed than in Lotus II). Still the second one have some great samples, like "TURBO" or "EXTRA TIME", or the unforgettable "YEEEHAAAW" when driving under a trailor.
Lotus III wasn't dissapointing, it had an excellent number of tracks, and the choice of cars was a great addition, as well as bringing the Radio back. I missed the samples from Lotus II though.
Come to think of it, I think it's a shame they never made Super Cars III too, as an alternative to Lotus III.