Going with Baby on this one. Although Omega had an association with the Dragon Balls, individually he was rather bland, and his power was practically the only thing that stood out about him. The concept of the Shadow Dragons and the consequences of misusing the Dragon Balls is interesting, but had plenty of room for improvement as far as the actual implementation. Although Baby isn't as connected to the series as a whole, his creation and goal still has substantial relevance through Saiyan-Tuffle history. Furthermore, Baby's craftiness and manipulation made him more engaging to watch, and his arc also marked the debut of Super Saiyan 4 which became integral to the series from then till the end. As far as battles, Baby also takes the cake since his final battle with Goku had some pretty good sequences and extended beyond brute force with Goku's use of psychological tactics. In contrast, Omega was either stomping his opponents or being the one getting stomped.