going to go over the main reasons why some babies i've ranked really low and others pretty high in response to the comments.
mongo baby: although he's a very unique baby and is only just 25 more hearts than brother bobby he's simply just not worth it in the rogue-like way, you can only get about 10-11 babies in a run + the 4 (5 with eden's blessing) starters and can carry only 4 (or 5 with schoolbag), as for his actual use you're only saving up at best 75 hearts since the most expensive shooters cost 200, even saving 75 hearts on each lane is just not worth spending a deck slot for an ability you have already chosen, just spam more chads, the ability also gets increasingly more difficult to use as minecarts, spikes and teleporters get in the way.
wiz baby: he's a good damage dealer but the attack type simply just makes him not worth it, you need to place him pretty far ahead because of his range and he can't defend himself alone, imagine a scenario where there's 2 gapers, one at the top lane and one at the bottom, 2 wiz babies in the middle kill them, but 2 brother bobbies at the far back would also kill them for 50 hearts cheaper, now what if the gapers are both on the top lanes? brother bobbies still do fine but because wiz babies can't pierce they'll only hit 1 gaper while leaving the other unharmed , i'd say only use if you have telepathy for dummies in your deck as well for more reliable dps.
lil steven and brain worm: i'm going to just put these together because they're not good for the same reasons, dps, you're spending way too many hearts for these when you could instead just get brother bobbies, resulting in more dps and cost efficiency, and even if you do get the same amount lil stevens have the same dps while brain worms have ever so slightly better dps, but not enough to justify the cost and a deck slot for the run, just about the only reason why they aren't in bottom tier is that they are quite good against bosses.
double baby: simply just not worth it, way too expensive for a heart producer and since in loops you're limited to a certain number of hearts and other babies usually don't cost too much it's not necessary to get a higher heart production than let's say 6-8 lil chads; now let's compare it to lil chad, abysmal in the early parts of levels as you need to wait for 2 heart drops before you get your first one down (1 if you have <3 and 0 if you use candy heart but even then why are you focusing so much when lil chads work better), it's not as spammable because of it's high cost so you can't just sacrifice them to sperm whales.
bbf: all i'm gonna say for this one is this, doctor's remote / kidney bean, dry baby exists, doesn't 1 shot sperm whales and stoners, can't be used against ragdoll and middle horn.
acid baby: reason why it's not lower is because of the newly added percs pill which gives a book of shadows shield, and even before that giving 8 babies power pill / speed up significantly increases dps, but it's still just situational because you need pills to have him work.
harlequin baby: same struggles that wiz baby has but not as bad, very good dps but there's simply just better choices for when it comes to late-game, absolutely useless on the top and bottom lanes, good for bosses at least.
snowball: it's a really good effect and very cost effective but unfortunately the enemies that you would want to use it against are immune to freeze and chill (stoner, hardy, frosty, sperm whale).
paschal candle: why is it worse than multidimensional baby??? multidimensional baby synergizes with ANY ground projectile that you use, while paschal candle has multiple counter synergies with it such as pegasus extinguishing the candle, freezer babies shots turn into normal tears, parasitoid's shots turn into fireballs that don't split into flies anymore significantly reducing his optimal dps; sure, multidimensonal baby also dupicates enemy projectiles but are you gonna really let frosties and hardheads get close enough to take advantage of that? you should rethink your strategy if so.
ghost baby: shoots slower and does the same damage as brother bobby, can pierce only up to 3 enemies, can't pierce through masks, bad single targert dps, gets completely outclassed by deserter.
mystery egg: not in the highest tier simply because it's outclassed by parasitoid in just about every aspect, it's still very very powerful don't get me wrong.
abel: simply this high in the list for how incredibly high the dps is, very cost effective and can synergize with paschal candle and multidimensional baby, unfortunately it gets worse the further to the right you place, otherwise it would of been in the highest tier.
spike baby: hard to believe but this is the baby with the highest single target dps in the game so far, just a simple row of defensive babies with these in front of them can unironically carry you through most enemy layouts without a sweat, also becomes even more insane with isaac's fork and sanguine bond, it was very tempting to place on the top tier but sadly can't hit flying enemies like flies, plums, sperm whales and frosties.
fistuloid: top tier because he has a base dps that's higher than brother bobby, has quite a lot of health for a not defensive baby and the death charge attack is just ridiculous, does enough damage to kill just about any enemy that isn't considered a tank, also is a good counter to gurgle if placed at the far left, i'm unsure if mongo baby can copy the death ability but if so, that's probably the only usefull use i can see mongo baby having since it would have a faster recharge.