I say it's a tie between (1) the "vanilla" Behemoths of FFX, (2) the chapter 2-2 boss battle (Popokarimu; the las plagas-infused giant bat/insect creature) of of RE5.
(1) I mean, really...seriously?? All the strategy guides and word-of-mouth saying these were "really BIG bruisers" were generally all hype. By the time you get to the area you first encounter them, your party is more than capable of crushing them in several turns, in spite of their (auto)buffs & crazy physical power. Their "King"-version at the end of the game, on the other hand...
(2) This is probably the most disappointing boss battle of the entire game. Even on the most difficult difficulty setting, this is pretty much just a game of "bait-&-switch" with the boss. The battle would be much more challenging if only 1 character was allowed to fight at a time. For a boss that really didn't have a respectable defense in the first place,and a [a href="http://tvtropes.org/pmwiki/pmwiki.php/Main/WeaksauceWeakness"]weaksauce weakness[/a] to bypass it entirely, it makes it almost a [a href="http://tvtropes.org/pmwiki/pmwiki.php/Main/FakeUltimateMook?from=Main.LevelFiveOnix"]overbloated glorified mook[/a]...