They're all interesting in their own ways.
Garth provides removal, draw/recursion, and big fat flying wincons - he probably includes the most versatility of any of the lot - but wants some self-bounce and haste, and lands to give you other utility since he only uses some of your mana. He also needs to reliably hit wubrg on turn 5 or ideally 4, which is tricky in 99 land, and extra R generation is good for shivan dragon.
Kenrith pretty much fills up your mana usage without as much need for utility lands, though he doesn't provide a lot of control options so you'll want those from lands. Mana fixing is looser since he only needs white to cast, making T4 a lot easier off Ancient Tomb or whatever. He provides a wincon primarily by pumping himself for lethal commander damage plus trample. Doesn't need haste since his abilities don't tap.
Sisay, Weatherlight Captain focuses exclusively on legendary lands. There are some solid options in that pool, though far from universal utility. It's the only option that majorly ramps without support, though, so it does provide some ability to accelerate your plans. She's a mana hog like kenny, but since she doesn't *do* very much on her own she still needs a lot of utility from her lands. As a wincon she relies on her lands, so manlands are probably very important. She's easy to cast, but her ability isn't, and you'll probably want to activate it more than once after you get to 10 mana, so a lot of fixing is important. She does provide fixing herself, though, which is nice.
Child of Alara provides the strongest control option, as a board wipe in the command zone. It requires sac outlets to reliably function, though. It's a mediocre wincon as a 5 mana 6/6 trample, but serviceable, and it blows away blockers for manlands etc. Fixing-wise, he's on par with Garth - you need wubrg but after that you're pretty clear.
Sliver queen is pretty boring. She poops out tokens and that's it. Still, it's a strong ability, however it does make utility lands less important and so doesn't give you much of a plan beyond just making tokens and hoping you get there. A few utility lands end up being crucial here, namely Vault of the Archangel and Gavony Township.
Najeela is similar to sisay in terms of mana requirements. She can snowball pretty quickly as a wincon, but a couple decent blockers under each opponent's control and you're pretty stuck. But if you have wwuubbrrgg to throw around, you can go from zero to hero pretty insanely fast.
A lot of the contents of these decks would be pretty similar, but there are definitely some cards which are going to vary. Also of note is that Jegantha, the Wellspring can companion for any of these decks with no additional restrictions.
Here's Garth, who I think is a pretty middle-of-the-road option in terms of deck composition.
99 Lands - Garth
Approximate Total Cost:
