I'm also curious as to what makes it so great but I think FE final bosses aren't *that* bad generally, at least if you go with the entire final chapter as the "final boss battle" instead of just the boss themselves.
Actually, if we're going by this definition, then FE7's probably a good contender. Nothing like a challenging, stat-boosted boss rush to cap off an SRPG campaign!
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http://www.youtube.com/watch?v=SBpfwpOPXNc&NR=1&feature=fvwp
I watch Planet Chasers Starlight Excellent all day, every day.