it really matters what you want.  it helps to keep in mind that these are all excellent figures, and it's possible to do great things with any one of them.

apollo is very versatile, poseable, free and very, very well supported.  apollo is a rock solid quality figure.  he's got great bending, good skin shader made cutting edge by bagginsbill, immense support and he's free.   you can use him for anything from a baby to an elderly man (see carodan's work).    i cannot say enough about his quality, or the generosity and talent of those supporting him.  he's incredibly versatile, and can pull-off everything from baby to youth to dwarf to superhero to sumo wrestler.incredible range, great flexibility, and by far the most technically advanced poser figure out there.

on the other hand, he's low definition compared to most figures. he's got incredible joints, but they're round instead of pointed (elbows)  or square/rectangular (knees).  there are displacement maps to help, but i still see people saying they couldn't eliminate his "puffiness," and to be honest i haven't seen anything that did to my eyes.

so if you're wed to either bony (as in bones showing, not  just thin) or really defined musculature, he might be a problem.   people who like very muscluar male characters, or even just craggy and lean ones, seem to have some complaints about not getting him to fit their style. so, just to clarify to the many apollo devotees, i'm just saying that he does have a style and that it doesn't necessarily suit everyone, and the people he doesn't suit seem to want more definition.    i'd bet he makes  great bishounen elf types, and i've seen him make all different sorts and ages of "every-men."  but if you want to recreate your favorite rob liefeld cover, he's not the easiest starting point.  in that case, you probably want the freak (who is so limited, i didn't include him in the critiques).

m3 is middle of the pack on all fronts.  not the greatest bending, but about on par with v3.  not the greatest musculature, but again about on par with v3.  other than a few stupendous custom characters for him, he doesn't really stand out in any area but support.  because he's michael, most male products are made for him.  this is really, really important when, for instance, one builds a scene with a great aery soul sci-fi outfit on a figure in a stonemason scene, and then one needs the female to have a male counterpart that doesn't look underdressed (or overdressed, depending on how you look at it).  the only outfits i know of that might work with 2035 girl or skorpio rising are for m3.  same goes for outfits to match  white rose and moondancer.  david (discussed below) has some possibilities, but only a few, imho.

david is better in terms of bending, morphing and even, imho, more masculine and realistic face.  and while he's more petite at default, like all the unimesh crew (and most figures, for that matter), his legs and arms scale just fine, and you can play with proportions.  he's unimesh, so he can take m3's textures.  but of course not his clothes, and while david has a decent wardrobe, he could use some help. 

and at all that, he's still only marginally better than m3.  while i really notice a difference, i'm not sure anyone who didn't use poser, i.e., your average generic gallery visitor,  would notice anything at all.  he's still unimesh, and prone to some basic unimesh flaws (joints!).  basically, he's an improvement (as a later model, this makes sense), but he's not a leap forward.

david is  good if what you want is a better answer than m3 to the sure bet of support for a daz figure. 

all of the above have morphs by capsces. capsces' morphs can truly make a difference between a figure being frustratingly limited and amazingly versatile.  and yes, the already incredibly versatile apollo has capsces morphs.  they look amazing.

then we have the eF generation 2 males.  i'm assuming you don't have poser 6 or any of the original males (james 1, koji).  the three share the same body which has bending problems i'm sure people can tell you about.  what the g2 males (koji, james, kelvin and simon) all have are bodies which are fairly defined and faces that are very individual and full of character.  personally, i haven't found figures with faces that have amazed me the way my first glimpse of miki1 and koji1 did.  and even james 1 i've made into some pretty interesting characters in the face room.  the g2 males still impress me facially.  but i personally wish their heads were attached to david's body (not apollo only because of the style and detail differential).  and while support of the G2 figures seems to be steady on the part of eF and eF affiliates, it's still tiny compared to unimesh support.

basically, if you're looking for figures with lots of individuality and character, and you're willing to work for your pose, check out the eF guys.   but don't if you want a blank slate to develop your characters on.

m2 is a version back, but still has some uses.  he was given away free as a bonus to platinum club members, iirc.  he also has one of the earliest capsces' morph sets (boris).  many (including myself) have a hard time morphing away his gi-normous default pecs (called man-boobs by some), but others have had no problem at all.  he becomes massive pretty easily, where the unimesh crew needed the freak to fill that role.  that said, his hands and feet stay tiny.  in terms of bending he's 2 generations behind v4 and probably 6 or more behind apollo. 

if you can get him cheap or free, he's probably worth a try (hard for  me to say only because i collected his freebies when he was current, not recently).  or if you really want,  to do defined superhero type physiques.   many people more knowledgable than i regarding musculature and anatomy put him at the top for that.  but his flaws make him comparatively limited unless you're willing to put time in.  he has some great characters, and people have done some wonderful stuff with him.  just as is true with v2.  and with that comes the codicil that they don't have any of the improvements to figures that have been discovered in the past several years.

then there are the p4 based guys who are also free. domus with his scaling and remapping, dphoadley's duke (remapped to take m3 textures), and i think there are few others floating out there.  to me, the interesting thing about these are they're low poly and have new techniques applied to them.  people aren't afraid to experiment with the old figures, and it's great to see what's possible.  it would probably be a good idea to look at p4 male morphs and such if you're looking to do all animation and are rendering with smoothing.   especially considering that modern dynamic clothes can be fit to practically anyone.

so, yes, apollo wins the all-around (hands down), but m3 wins best supported, david is a runner up on support and surpasses m3 on technical, g2 males win most personality (out of the box), m2 wins the body-building award, and p4 male solutions win the lightweight and indie award.   

that's my synopsis.  i'm sure others will have other opinions.