A600KiloBear: I like your candidates, especially R’ Yakkov Horowitz. But I would like to add a few more:
R’ Noach Weinberg ZT”L, for his work on stopping the “silent holocaust” of assimilation by bringing kiruv into the focus of mainstream judaism. Although he has passed on, his work continues through his organization Aish HaTorah, and through grassroots organizations like Project Inspire (the people who brought you kiruv.com and the brand new seeyouonshabbos.com).
Rabbi Doctor Abraham J. Twerski, for his work on bringing all sorts of sensitive and vital problems (and solutions) to the forefront of mainstream judiasim.
Rabbi B. Shafier, for providing applicable mussar shmuzim to the jews of today, available for free (audio/video, stream/download) at theshmuz.com.