X1's sigma why? because of the smirk he has when he wall jumps, because of the freaky sound you hear when he beams up his saber, because he's one of the few, if not the only mildly challenging stationary boss.
Didn't like X2's sigma, tho I completely drooled over that he was using Electric spark, X3 was just downrigth atrocious...X4 was...tripping, not really my taste, tho the Laser Scythe was pure win. X5's...liked the purple mantle, liked the RMW4 Wily Machine reference, but other than that...no, X6...was just outright scary, he still gives me the creeps when he laughs after his first body is destroyed...X7 was too colorful and nuts, but the second form was epic, one of the more impressive final battles imo...X8 was just satanic, but not too special, getting his sword was a treat tho. Oh, and before I forget, Soul Eraser's Sigma, his second form was quite impressive, and one of the few sigma's with a OHKO (I think.), still none of them has something on the first ^^.
-Elpis