As has been said before Sunkern has the lowest stats. However, it hasn't fully evolved. Pokemon that have fully evolved/don't evolve, and have low stats/have hinderances are:
Shednija: Low total base stats, mostly because of only 1 HP. However, Wonder Guard makes up for that somewhat.
Smeargle: Once again, low stats. However, with Sketch, it can learn any move.
Ditto: Low stats, but Transform takes care of that.
Delibird: Low stats, and you can't get it any good moves through level up.
Luvdisc: This has always been a bad Pokemon.
Slakoth: I know this guy is not fully evolved, but I just wanted to mention him. Truant makes him much weaker than other Pokemon, even those with lower stats.
Delcatty: Low stats for a fully evolved. Also, it is even worse if it has Normalize as its ability.
Regigigas: Although a Legendary with high base stats, it has a horrible ability that really hinders you.
Phione: Considered the weakest Legendary (although in my opinion, that honor goes to Regigigas.) It has the lowest base stats of any Legendary.
Unown: In my opinion, the worst Pokemon. It has low stats and doesn't have anything to make up for those stats. It can only have one move, Hidden Power, which is unreliable unless you know how to use it (I don't.)