Toroom - I Am Invincible! (Final boss.)
4Win or lose, I wish this roguelike listed what items you had at the end of each run. All I know is that my defense was buffed out the wazoo. Plus I had this homing shot that made this phase of the boss trivial.
On that note, let's talk about weapons. There's a bunch of 'em, but the game also showers you with them. Outside of boss rewards, you're likely to find the same weapons every run. There's also only three levels, so runs can feel a bit samey. If you can get a good seed like this one, though, it's pretty enjoyable.
This was the Hard mode I beat, but there's also an unlockable Fiesta mode. It has a larger emphasis on items. Bosses will even drop a bunch of them, whereas they normally just drop a random weapon. There's an unlockable character who benefits from the mode -- there's some tradeoff items that buff + debuff you, and he's immune to their debuffs. Those items are rare in the other modes, though, and he's got lousy stats, otherwise.