People are still remembering 2.0 bards with rose-tinted glasses.
Fine I'll play XD (I don't do PVP so no opinion there)
Bard's AOE is okay, and its single target burst damage is VERY decent with all the cooldowns available, but it falls off very quickly. The best thing about bard is the opportunity to upkeep its top potential DPS due to its mobility. That's why some bards who keep their DPS up at all times can outdo other jobs' DPS. DPS potential doesn't mean shit without the work of a good player.
BLM's AOE is insane, miles ahead of the others, and it's absolutely sustainable as long as you keep throwing mobs in their way. Their single target DPS is less than that of DRG & MNK, but still good enough to maintain overall good DPS throughout a fight. Since BLM is such a big hitter, that means every time you miss a cast, your DPS really hurts for it. Great BLM DPS can fall off very quickly due to shit mechanics, so good BLMs have to think dynamically about how to upkeep their casting.
DRG (my main) has good AOE, but its sustainability is poor due to TP consumption. DRG AOE is good if he has another good AOE job to help kill multiple mobs quickly; if it drags out, he's screwed. DRG's single-target burst is very good, maybe the best, and its single-target DPS sustainability is very good, but not as great as MNK's. Both melee classes can do very well with single-target mobs with dynamic mechanics (like Titan) due to their mobility, but can potentially suffer when made to move around too far too often. Still their mobile DPS upkeep is "better" (I should really say potentially easier) than BLM, but nobody matches bard in that department. DRG's multiple buffs and off-GCD abilities allow it to really shine in DPS checks.
I haven't played high-level MNK, but have played with one in my static for months, so I have a pretty good feeling on it. MNK AOE does not match up to DRG's (which doesn't match up to BLM's), but it's single target DPS sustainability is unmatched. MNK's DPS will only fall off when he's not allowed by mechanics to keep up his buffs.. which I guess you can say for any job. If a DRG is a Ferrari, a car that's fast as shit but will spend half its time in the shop (waiting on buffs); a MNK is a Corvette, plenty fast, and it'll get ya to work everyday.
I haven't played SMN at all, but it seems like a caster monk (slow and steady) with superior AOE. It also sustains DPS more easily in mobile fights than its caster brother, BLM; when combined with flexible DPS spread via bane, it can be king of very dynamic fights. A melee will have to work very hard to keep comparable DPS with mobs coming and going throughout a fight, and will still probably fall short.
At the end of the day, you really can't go "LOL MY JOB IS BEST!" coz it'll only be true some of the time. I would say that MNK and SMN would be the "best" overall DPS, but I know I really like having the other jobs around when burst DPS is required. I've gotten my party out of many hairy situations as the only available DPS due to mechanics (conflags + death) with both BLM and DRG, whereas a MNK or SMN would have been shit outa luck.
This game has been designed and meticulously maintained to be very balanced, you want different kinds of DPS around to have the overall best party. I would like to have all DPS classes leveled and usable in endgame content, just so it's a no-brainer when it comes time to enter a particular fight. I would say it's best to do a DOT and burst job, but I simply can't resist big pretty numbers, so I mostly play DRG and BLM. I do want MNK in my arsenal though, really badly.