The tuatara is the world’s only extant reptile? Did I miss some sort of mass extinction event last Thursday?
I think Forest Elephants were recently put into a separate species.
The genus Camponotus (Carpenter Ants) has over 1,000 extant species (about 1,023, not including subspecies, by my count…), so it’s definitely up there in terms of species:genus ratios.
Of course, if you want the fewest, the winners would be all the extinct genera, which, by definition, would have zero extant species 
Unless we can find some genera with negative species…
Are you using the “new” Acacia, or the “old” Acacia? The “old” version was found to be polyphyletic and has been broken out into several new genera, with Acacia now only representing those Australian species (and a couple dozen other ones), which reduces Acacia proper to about 967 species or so.
Creationists can find some with a value of i 
The taxonomic groupings you mention are generally orders or families rather than genera. Spiders comprise the order Araneae; beetles are the order Coleoptera; crabs are an infraorder of the order Decapoda (which also includes lobsters, shrimp, etc); elephants comprise the order Proboscidea; orchids are the family Orchidaceae. The only group you mention that is actually a genus is pines, Pinus.
The largest order of all is the beetles, with 350,000 described species, and probably millions when undescribed species are included. Beetles comprise about 25% of all described life forms.
Orchids are the largest family of plants, with between 22,000 and 25,000 species. The orchids may make up about 10% of all species of flowering plants.
On the low end of the scale, there are a couple of animal phyla with only one species, Placozoa (although genetic analysis suggests there may actually be about 80), and Micrognathozoa (although some consider this to be part of the Gnathozoa). Among plants, the familiar ginkgo, Ginkgo biloba, commonly planted on city streets, is the only member of its division (equivalent to an animal phylum).
They were, but this has been disputed.
Quoth psychonaut:
D’oh, obviously I should only have gone up to order, not class.
In my defense, I did indicate my comments on maximum species per genus were WAGs. I presume Poa has been reclassified, with closely related species telescoped into one (or else my memories of how many species are included in Poa were very much in error).
For anyone interested, Wikipedia on Loxodonta.
As I think DrFidelius was getting at, it’s not (entirely at least) a question of biological fact. There is an arbitrariness to what gets classified as a genus, and to what taxonomic level is assigned to a group (genus, family, order, etc.). Species has some claim to being a “natural category” (at least for sexual organisms), but the others are the creations of people. It’s a bit like asking what the largest island in the world is, and hearing that it’s Greenland because Australia is too big to be an island (I realize there may be other reasons for disqualifying Australia, but I think the analogy still works).
Actually, there are comparatively few described species of bacteria in general. There aren’t many morphological characters to base species on. Genetic studies have revealed a great diversity of strains, but then the question arises of how different strains must be qualify as “species” in asexual organisms that don’t fit the Biological Species Concept. In any case, the genus Bacillus itself has recently been split into several different families.
I would have guessed Rattus myself, but it has only about 57 species. The largest genus of mammals turns out to be Crocidura, the white-toothed shrews, with 175 species.
IIRC, the largest bird genus is Zosterops, the typical white-eyes, with about 78 species.
The largest living thing in the world is so big that not only does it have its own species, it has its own genus. Sequoiadendron giganteum.
The Drosophila may have them beat. The Tiger beetle genus Cicindela is another massive one. I’m sure there are others I’m blanking on as well.
Your information is outdated: The current title-holder for world’s largest living thing is a humongous fungus. They’re just not as obvious, since they’re mostly underground.
The ant genus Pheidole also has around a thousand species, but Drosophila beats that.
Some orchid genera will beat that, as well as most of the other candidates above.
I was just about to mention Bulbophyllum! Solanum is another big one, with ~1,400 species (though obviously outdone by others mentioned).
Funguses are evil, not living. Eww. Yuck. I stand corrected.
KneadToKnow:
Oh, I wasn’t correcting you. I was just interpreting the OP’s question to mean “what species is the most taxonomically unique”, and I figured that one extant species + no fossils within the same genus would beat one extant species + fossil species within the same genus.