Squirtle, Zapdos, and Mewtwo are at the very top of OU. With those three alone, you could beat the elite four the first time in under 4 hours and probably beat it in the rematch in at least under 10 hours (I haven't tried that part yet). If you ultra ball zapdos (not too bad, most of the time is in resetting), you can keep the master ball for mewtwo.
If you have to reset so much to catch something, that raises the question if it's worth the time and the annoyance in an efficient run. That doesn't apply just to the legendary birds, but Snorlax as well.
I used Snorlax in my recent run and was very dissatisfied with the results. For one, he's slower than pretty much anything, which is a big con in-game. Not only do battles take longer to finish, but sleep, paralysis, accuracy falls, critical hits, flinching - all of that may prevent Snorlax from getting his quick kills, and when you think about it, there are only so many enemies that Snorlax can fight without being afraid of the aforementioned annoyances. Sure, with that bulk, he'll never die unless you send him out against a fighter, but I'd prefer to just sweep through things quickly.
I'd still say that Snorlax is the best Pokemon for Sabrina's gym, assuming you teach it Shadow Ball (which is a substantial investment).
I'd also argue that Pikachu needs to reside in the Top tier. Good attack, special attack and speed, takes on earlygame Pidgeys and Spearows very well, plus for non-Bulbasaur starter teams, Pikachu is pretty much the only hope against Misty's Starmie who's very tricky to take on at that point (unless you miss the gym and come back later, which is very uncool to do).
Sandshrew needs TMs in order to be good, and while you can have as many Dig TMs as you want, he suffers in usefulness because Earthquake is so far away, and his attacking options are very limited.
In general, I think Psychic-types reign supreme in FR/LG, and Ground-types are less useful than in RBY. Levitate Koffings/Weezings are all over the place (most Rockets and Bikers have them on their teams), not to mention Gastlies.
I think for that reason alone Abra and Mr. Mime should go to Top Tier, and Jynx should be at least in Mid.
It's not necrobumping if you have something to add (which I hope I did). I found this by googling for smogon in-game tiers and wanted to discuss the games when I saw this. I think people are eager to participate in these threads from what I've noticed on these forums, but if you don't care to participate, nobody's forcing you to join.
For Sabrina, I think we can agree that Shadow Ball is a good TM for this gym. Snorlax learns it, and some other bulky Pokemon who could use it for the Psychic gym are Flareon (specially bulky, and his Shadow Ball hits the hardest) and Clefable (whom you have a lot of time to train).
I also think Agatha's strange physical ghost movesets shouldn't be underestimated. If you're not overlevelled, your Psychic of choice will have no chance to attack her Gengars - Shadow Ball is a sure OHKO for Alakazam and Mr. Mime, most likely Jynx as well.
That guide is already on site and unfortunately, there's no more discussion on the issue. Also, what TCL is trying to say without being an ass is that you shouldn't just leap at the chance to necro the first thread you see. I googled "frlg in-game tiers" and the first result leads to the C&C thread that turned into that very article. The link to this thread states that it was last updated 2008. It would be better to go to the most recent links. Play it safe by not bumping such an old thread, no matter what you have to contribute to it.