There's no such thing as a bad tournament, if your looking across every player on the tour. But most to least prestigious is hard. I'd say:
1. Slams / Davis Cup (both equally mean a lot IMO -- making the history books, and a chance to make your Country proud -- both highly memorable for the player)
2. Olympics (only happens every 4 years -- also mean's the world to win Gold; again doing something for your Country and being remembered for it)
3. World Tour finals (still great, but probably NOT as likely to be remembered in the history books as above -- but still memorable in a way of solidifying yourself as one the world's best)
4. MS1000 (fairly large tourneys, not likely remembered to be something huge, but they give valuable $$$$ and ranking points -- essentially more of a "springboard" to do well and have potential for the more prestigious tourney's listed above)
5. The rest...
Apologies for my essay, but that's my opinion