White Mage for sure - earliest access to critical healing and removal spells, like Vox and such. Time Mage has some neat tricks early on, but not enough for a first class.
I would probably make Penelo a Monk first - build up her HP reserve first. Why? You don't really get amazing black spells nor near enough MP to actually use them for awhile. Now, once you do (where I am at level 29 or so for her she's dropping spells that hit an area for 3k or more damage, more than triple what one of my melee guys pull off, and she's got Chanelling 3 and a Sage Ring so said spell takes like 8 of her 380 MP, and then she gains a little of that back from Warmage (gains MP for dealing damge with spells) and Headhunter (gain MP for killing an enemy).
Plus at that point you'll have many more gambit slots you can use to set up her magic, for things like "if Fire-Weak: Firaga" or "If 2 or more enemies: Bio" and such.
As a Black Mage early on she didn't have any problem doing her fair share during fights, but once she was out of MP I'd have to swap her out until the next Crystal or use Ethers, which I was loath to. I paired her with Red Mage though, not Monk, but I can see early game Monk being a great fit for late-game Black Mage.