Cecil Turtle is a minor character in the Looney Tunes franchise. Though he has appeared only a handful of times, he holds the great distinction of being one of the few characters to beat Bugs Bunny at his own game. Slow and steady wins the race, as they say. And they don't come much slower than Cecil.
Mel Blanc was the first to voice Cecil, making his appearance in the series of Bugs Bunny shorts. Mel's voice for Cecil seems to be a slower version of Barney Rubble's classic drawl. A perfect match for a "slow" character like Cecil.
An uncredited Frank Welker voiced Cecil in his appearance in The Sylvester & Tweety Mysteries. Frank's slow drawl (a voice we don't often hear him do) is a good match for the character, though I find that there's the slightest hint of "cowboy" in his voice - like he's doing a slooowww Jooohhnnn Wayyynnee for some reason. Maybe that's just me?
Jim Rash (best known as Dean Pelton from Community) voices Cecil in The Looney Tunes Show. Cecil's portrayal in the show is a little different than the other two. He appears to be a lot more sarcastic and passive-aggressive, but still retains that slow, drawn-out voice.
All four VAs are good, but my vote is going to Mel.