I'm kind of torn between BF23-26
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Whichever one Elena joins on, I don't recall.
As well as BF28 and 32. 28 for the story-based reasons, especially BF28-3, and BF32 because of the insationable challange it provides. Sure it'll leave you screaming at your Gameboy at times, but once you get past it, you can't help but notice how fun it was to finally trounce that hellish battle.