I chose Late Night for you since this was the EP that finally allowed for players to have more community type lots they can build rather than the old stand by gym, museum, library etc. If you were strictly playing the base and one add on EP. This EP opens up gameplay to the core, and adds a chemistry system between Sims that is not as well understood as the attraction system in TS2. There is also a perk object (I can't remember) that can be used to keep community lots open passed the 2;00 A.M cut off etc. I was not thrilled with the hot spot setup (because it was lame) but it was the first EP to allow other type community lots to be built. It might not matter so much after the later EPs I guess.
Generations would be the other one, since Sims AI is improved in more ways than one. If you didn't like the lack of interactions in the base (compared to TS2) at the time then Generations adds many hidden things that you might notice right away with the Sims being more interactive with each other compared to just standing for awhile on community lots not talking to each other etc. Generations has hidden gems that increase the AI to seem more compatible with TS2's AI.