English Translation
We cannot respect such a great man enough.
You can also replace 偉大な人は with 偉大な人を, but は is preferred in this context.
は as 助詞 (particle) is sometimes used to emphasize the difference of two things/situations. In this particular sentence, the speaker picks up the great man, and distinguishes from other ordinary people by saying “such a great” in English. So, you’d better say 偉大な人は.
But please bear in mind that は is also attached with not only an object, but also a subject. So. このような偉大な人はいくら尊敬してもしすぎることはない。can also be interpreted as “Such a great man cannot respect for something/someone else enough” (= This great man is always very polite and humble). You cannot judge in which way は is used. You need a whole context.