Rydia - Her summon quests are more story-based, I'm not a huge fan of "superbosses" anyway, and I also think FFV is easily the worst of the SNES FFs. The powerful summons of Leviathan and Bahamut are well worth the trouble and give you a clear sense of just why Zemus wanted to take out the summoners.

By the way, I'm pretty sure the Sylph are not mandatory and that I've even beaten the game without collecting them before.