Not anime series this time. Here's where you get to have your say in which anime characters have the best and worst Voice Acting in dubbed versions.
My list is gonna start off with one of the more recently discussed animes in this particular Forum; DBZ. Piccolo and Vegeta are the best, with the VA's (especially in the later eps) pulling off voices that are very well suited to the characters. Frieza and the Ginyu Force (the latter from before the entire VA cast was replaced, except Ginyu himself, whose new VA was quite good) are also quite good. Goku's dub was one of the bad ones, though the complete change in VA cast halfway through the Namek arc actually improved that somewhat. The rest I find somewhat mediocre - not bad, but not really good either.
Tenchi Muyo is a rather well done dub, with most of the western VA's varying from good to great. Characters that stand out somewhat are Ryoko (I haven't seen the third season, however, so I can't comment on the change there), Ayeka and Tenchi himself. Kagato was a bit of a letdown after hearing the original Japanese (damn that echo effect made him sound cool ^_^).
Those are the dubs that I know better than the rest. Other memorable ones for me are Spike and Jet from Cowboy Bebop, Duo Maxwell from Gundam Wing, Ryoga Hibiki from (you guessed it ^_^) Ranma 1/2 and Bear and Mimiru from .hack//SIGN.
Other badly dubbed characters include Ranma from Ranma 1/2 (eps 1-64) - where his male voice was done by a woman so when he was male he still sounded, you guessed it, like a woman... Hikaru Shidou from Magic Knight Rayearth - I only heard a bit of the dub but the way her voice was done grated on my nerves for some reason so I switched it back to Japanese with subs, and Sepiroth from FFVII: Advent Children - they really should have tried just that little bit harder to find someone who could do an English voice that has the same ominous sound to it that the Japanese one did...