I looked through this thread and I'm probably repeating some things but I think they deserve to be repeated.
Chansey in UU is obviously right up there. It's basically a mini-blissey which is really stuck with Seismic Toss, so it walls nicely in UU.
Porygon2.. What can I say? I use it as a lead some in UU myself, it's quite good at the job. It can recover to stay alive more, gets Discharge and Ice Beam off a good Special Attack, and Magic Coat is good when it starts to fire status back at the user. It doesn't bounce back Taunt but it's still able to attack so it's not too much of an issue. Also nice for Trick Room teams.
Rhydon making NU really brings a monster to that tier, it hits hard in UU, but in NU, where there are some powerful rock-weak Pokemon, Rhydon is deadly. It gets Rock Polish, Swords Dance, STAB on Rock and Ground, and a high attack stat. Sure, it loses to special moves and has a ton of weaknesses but that doesn't change its upside.
Magneton is actually a great steel trapper for the lower tiers. It's faster than Magnezone is in OU, so it'll outspeed and Magnet Rise on more. It also has a surprising special attack stat to run off of as well, so it can rise up and revenge things like Aggron and Probopass.
Piloswine seemed like nothing to me at first. Then I tried using it in hail like HeysUp did, and I was surprised. It works like Mamoswine in OU, it's not like we're breaking new ground here, but down in NU, it actually packs a punch and has decent survivability thanks to the HP stat.
Last I'm bringing up is Munchlax. I noticed in the NU thread that people were talking about Curse Munchlax being used every once in a while over Lickilicky due to the higher Special Defense, and I used it a couple times, and it really works. After a Curse it'll pound things as well as most evolved things, while taking hits from both ends quite effectively. It's amazing to mention a baby Pokemon in here, I know, but ignore the label and notice the surprisingly good Pokemon in Snorlax's huge shadow.
I know I didn't bring up Dusclops and Trapinch and all, but I need more experience with them before I comment on those.