Garchomp - OU
Latias - Cant decide, really.
Manaphy -Uber
Garchomp - Garchomp is hard to use, yes he is. The scarf ones mostly need Outrage to do good damage, always to get met up with an Ice Punch Jirachi, BP Scizor, HP Ice Mzone etc. SD Chomp is not nearly as good as it was, since Scarf Heatran, Scarf Magnezone, (Scarf) Chomp, Latias, Gengar, Scarf Jirachi and Scizor are everywhere, so it is really hard to pull of a SD and kill 2 or more Pokémon. When Chomp is in town, everyone will use a max SpDef Skarm, for good reasons. There are several ways to win against chomp, locking it into Outrage and revenging it is the most easiest way. Chomp needs Outrage to get some essential KOs. Skarm + (t)spikes damage works as well. It doesn't work that good in the steelbased metagame.
Most people already are using BP Scizor, Scarf Jirachi etc, so you don't have to change your team that much (I didnt have to) to beat Garchomp. It is kinda unpredictable and uncounterable, but hey, same counts for Pokémon like Salamence and Jirachi.
Latias - We all know its ups and downs, pursuit weak is his onliest weakness. Latias easily shuts down most kinds of stall. Though, it has the same problem as Gchomp. Its hard for Latias (heck she even doesnt have strong ground/fire move's) to stay around in this steel based metagame. Scizor, Metagross but even more Jirachi are some of her biggest pains, and those are everywhere.
The Trick version requires quit a bit prediction, since most of her ''counters'' are choiced. Some pack Pursuit (TTar, Mgross, Weavile, Scizor), not much to say.
HP Fire.. that.. has cost me a lot of battles. The 30Spd IV really sucks!
Manaphy - Manaphy is a total pain. She does so well in this steel based metagame. Gigantic movepool, awesome allround stats and usefull ability makes this thing really hard to combat. In my battles, I felt that I really needed a solid counter for this thing, otherwise it would walk over my team. Manaphy is a Pokémon that ''rapes'' today's metagame.