I don't know that I'd fear anything, but I'd be wary of:
Paladin/Sorcerer - Maxed Charisma. This is, I think, scarier than the Fighter Cleric insofar as there is less wasted overlap.
Druid/Monk - I'd be at least a bit wary. Maxed out Wis and Int, and depends on Wild Shape and buff spells for Physical Stats. Adds maxed Wisdom to unarmored AC while in Wild Shape. No, you won't have maxed out BAB, but massive strength compensates for a great deal. Remember, just because wild shape forms may have natural weapons doesn't mean you can't make unarmed monk attacks instead. The Animal Companion also adds a flanking partner - which also helps compensate for the lower BAB.
Fighter/Rogue - Think of it as a Fighter - with great skills, good Reflex save, Uncanny Dodge/Evasion, and Sneak Attack.
Fighter/Monk - Think of it as a Monk - with better HD, BAB, and Fighter bonus feats.
Ranger/Monk - A great unarmored two-weapon fighter.
-Stuart