I actually also think Thomas Dekker did his job pretty well! In part because he's probably the most theatrical and mature voice actor Littlefoot ever had.
As for his German voice actors, Kim Hasper and Ozan Ünal, in I and II respectively, did their job the best. Ünal is also notably the only pubescent voice actor of Littlefoot I'm aware of, which gives him a more mature quality missing from most of his other voice actors.
Also fun fact: In Germany, Littlefoot actually has a female voice actor, specifically Rubina Nath, who voices him from VII onwards.