There aren't many terrible units in PoR. Barring RNG schenanigans, most units are usable.
Makalov is usually seen as one of the worst.
Many of the laguz are bad.
Sothe sucks in PoR: he can't promote because of story reasons, and is very weak.
I like Bastion because he looks so much like one of my old friends, and he seems like a cool guy, but he's generally seen as unneeded since you have Soren.
People who train Rolf swear he's amazing, but he starts out so weak... Shinon joins a bit late but is really good, so it's up to you.
Most people use Mist as the designated healer, since she has certain perks later on that make putting up with her low starting stats worth it. But, this also means not a lot of people use Rhys. Rhys is fine, though. Way better than the Bishop you get much later...
You can't get Pelleas until your second save file. He's not necessary by any means, but it is entertaining to have a dark magic user (he's the only one).
I don't remember if Tanith is worth using... I know you get Marcia way earlier. Sigrun joins way late, but has that cool summoning green units ability.
Geoffrey and Lucia are usable but generally not needed. They join pretty late. They have good stats, though, if I remember correctly.