Never really thought about, which wonders are "worst" or "best", much of that really depends on the situation. (On an archipelago map, where you intend a fast military victory, the Lighthouse may be the "best" wonder, enabling a victory date a couple centuries faster than anything else you could build instead...!)
But now that I think about it, I think I never built Sun Tzu's myself. If I capture it from the AI, it's nice to have, as it saves 1-2 handfuls of gpt and may allow upgrading&healing units right at the front line (a military game should be decided before railway), but I wouldn't build it myself. I'd rather take 8 Knights or 20 Horsemen instead.
So not sure, whether I would call Sun Tzu's the "worst", but it's pretty low on my priority list. However, I in fact built it in one of my recent 20K attempts, simply because the AI was so slow - both in research as well as in building wonders (for some reason they started their wonder builds either in corrupt size-1 towns or I managed to capture their core towns, before those were able to finish anything...). As you know, there is the long "wait time" between Universal Suffrage and the juicy modern wonders, where no new culture building becomes available for a long time. I was able to build it during that time, since no one had completed it yet.
Another point, why Longevity is in fact harmful instead of useful (unless in 20K): first of all, by the time Longevity can be built, all my cities already have reached max size long ago! Usually some time in the industrial age, my workers (and 100s of captured slaves) are done with building rails, and then I only keep a few stacks for pollution control and add the rest to the metros. So Longevity is pretty useless, as my cities have maxed out approx. half an era earlier...
And then it is harmful, because it kills We-Love-The-King-Day! As already outlined earlier, with Longevity there are inevitable a long periods with negative food, and when a city is starving, it loses WLTKD. This significantly reduces commerce and may make it difficult to maintain 4-turn research! (Especially on higher levels.) So when going for a space victory, I avoid Longevity, even if I could build it, e.g. if Genetics can be traded from an AI. (Of course in a space race game, I wouldn't research Genetics myself, as it is not needed for the spaceship.)