Having given my Watsonian answer to the OP, I'm now going to give my Doylist answer, and one I see at a lot of tables (if this isn't what happens at your table, that's fine, ignore the remainder of this post).
Target the Fighter, always.
The Fighter generally has the highest chance of survival due to their large hit point total. Other characters might be able to use their abilities to get him back up, either by money or even having a Rogue steal money to pay for a resurrection*, scout ahead to avoid enemies while the party is weak**, etc. etc..
Adventures can quickly come to a screeching halt when a player character drops to 0 or worse, dies. Which character this happens to can have a knock-on effect whether your adventure continues or ends right there. Certainly, no adventure I know of*** has ever been written to accommodate losing players mid-adventure. The GM either softballs the remaining challenges or just accepts that the mission will fail and it might even lead to a TPK, rack 'em up and let's play again!
So if you are averse to this sort of thing, picking on the guy least likely to die, and less likely to have resources that might be able to get the party to safety or get the fallen character back in action. And that's usually the Fighter, in my experience.
*Unless you subscribe to a "death is cheap, I totally have a good spot for your next character to appear in a timely fashion, sure, animate the pile of dead Bards with your new Necromancer PC" style. Don't take offense if this is your preferred style, please. I'm simply trying to evoke some levity here.
**Assuming stealth is actually functional in your game, skulking around isn't just an invitation for a solo Rogue TPK, and you allow characters to actually flee rather than be forced to stay in initiative order with a good chance the bad guys can catch them. Again, don't take offense, please.
***If anyone reading this always designs their adventures to continue running smoothly when missing 1, 2, or more characters, I applaud you. It's just not something I am familiar with. I've attempted to do this in the past, and it usually either fails or falls flat due to an obvious deus ex machina.