The most outstandingly note-worthy ones i can think of are:
1. Xiaofeng
2. Zhangwuji
3. Dongfangbubai
To further narrow down to measure combat prowess,
some further Criteria would be:
a. Must have Fought 2 or more Greats over a duration of time without being hurt.
b. Cannot have been defeated/injured by any single individual in any combat incidents before and after.
c. Dispensing of higher internal power to dispel another individual 's internal power does not qualifies as in the case of Sm, the Shaolin Janitor that is no a fair indication of martial prowess.
d. Sole reliance on objects , big or small, such as swords or needles to translate one's internal power without any actual direct battle contact. Eg, use of HIS or Dragon Sabre applies as in c.