Bon Voyage by far. The vacations locations are great and I love collecting the mementos. Bon Voyage is one of my favourite packs in the entire game. Like Dijktafone, it gives you an experience you can't truly get in The Sims 3 or 4.
Out of the other two, I vastly prefer Pets, because I actually use the pets, and it's certainly better than The Sims 4's embarrassing iteration of the Pets pack, Cats & Dogs.
Seasons, on the other hand, is my least favourite pack in The Sims 2 by quite a margin. I find Seasons just lacks so much, when I've been spoiled by The Sims 3 and 4 iterations of the pack. It adds little that I actually use - I dont use fishing or gardening in normal play, and the weather effects feel like no more than an aesthetic change. Not to mention Fishing and Gardening are in The Sims 3's base game and are more fleshed out there. There's basically no holiday-based content with the exception of Baby New Year and Santa Cookies. What is the point in a seasonal cycle if you're not going to add holidays? Animal Crossing had a seasonal cycle and holidays several years before The Sims 2 Seasons came out, Harvest Moon too. Basically every game that predates The Sims 2 with a seasonal cycle makes better use of it than The Sims 2 does. As much as I love The Sims 2, Seasons is just bland.
So yeah, TL;DR Bon Voyage > Pets > Seasons.
