For me, it is Amjad Khan as Gabbar Singh.

If you ask which is not.......it wud be better a way... as almost every charecter of it was a huge hit be. it gabbar singh or his assitants daakoos sambha and kailya, be it thakur...or his two heroes jaidev and viru,be it basanti or her tangeki ghodi ...dhanno or her mausi, and the radha thakur's daughter inlaw a vidow,....the silent love intrest of jai be it reham chacha or his son ahmed be it soorma bhopali or angrezonke zammaneka jailor ,be it helen or the jalal with that all time hit mehbooba mehebooba wat is not that famous frm that legendary moovie of indian cinema.........!!
and more over the dialagues are ever famous and till date it remains the only moovie which has got almost all hit dialagues watevr they uttered as dialague was a hit...
gabbar singh: "YE HAATH MUJHE DEDO THAKUR" AND " KITNE AAMI THE SAMBHA"
thakur: "NAI........... NAI"
veeru: "MAUSIJI AND GAONVALOAN........."
jaidev: "TUMHARA NAAM KYA HAI BASANTI"
basanti: 'CHAL DHANNO AAJ IZZAT KA SAWAL HAI" AND "YU ...more

Every character in this film is famous and have equal importance

I would go for Veeru

Dhanno and other horses

All the characyters of Sholey are well etched in the mind of those who have seen sholey! I still remember a nameless character who answers his friend' question "Jab angrez marte hain, usse sooside kehte hain."

Thakur

The film itself is the best.

Every character of Sholay is unique. Those two friends Jay and Veeru, that wicked Gabbar, that sweet blabbering Basanthi, you can go on stretching the list. Each one is unique and memorable.

I think all characters of Sholay are famous even today. But the most famous of them all would be Gabbar Singh. Let me put it in another way - remove Gabbar from Sholay and it will not work. The way Amjad Khan has potrayed the role of Gabbar Singh is not just outstanding but immortal.