1.the only后面用that没错.此处省略了that,还原应该是he is the only one for whom "that" I have my most respect.定语从句中that可以省略.
2.此处的for应该是respect for Sb中的介词,介词提前了.所以此处的For后面加的应该是人称代词,而不是物主代词.因此应当为“whom”为而不是“which”.
这里你应该理清这句话的结构：主语：he,谓语：is,宾语：the only one,定语：I have my most respect for.不用从句就应该是：He is the only one.I have my most respect for the one.