Bard
Cuz it's the only one that's not basically a direct variant of the others. And even then you could argue that its a type of Thief-Mage hybrid.
I was about to post similar, but not that it’s not a direct variant of the others, but because; at least from 3e on; it’s settled into the role of “Jack-of-all-Stats.”
- Better weapons than everyone but the Fighter.
- Better skills than everyone but the Rogue.
- Healing magic, but not on par with the Cleric.
- Enchantment/Illusion magic, but not as good as a Wizard.
It also sets itself apart by its magic being Charisma-based and spontaneous/spells known vs. prepared; making its casting feel legitimately different from the other too (more akin to tricks they’ve picked up in their travels than something they’ve rigorously studied the principles of or devoted their life and faith to). They also have party buffing by their bardic music and being exposition device via bardic knowledge as some of its own shtick.
They also make a good choice for 5th member because they can support/fill-in when other members are missing or debilitating. Does the fighter need someone on his side/back to keep from getting flanked or to be a distraction while the rogue sets up a sneak attack? Bard is decent enough in combat to do that. Did the cleric just get dropped by damage? The bard has enough healing mojo to get the cleric back up so you don’t lose all their healing resources.
Basically, they don’t shine at any one thing, but by being decent at basically everything they’re as asset at supporting and filling in the gaps.
Also, if you allow a little creativity in terms of what constitutes a performance for bardic music (ex. oratory) they also work rather well as a stand-in for the 4E Warlord class (just pick your spells from the more subtle end of the pool and say they’re “advanced motivational/demotivational techniques” you learned while studying at the bardic College of War).
TL/DR; Bard, because they don’t step on any of the core four’s toes and are a Jack-of-All-Stats (master of none) with a few unique knacks of their own.