Depends on if you’re looking for just normal Challenge Mode runs or if you’re going Time Attacking- the former usually favors survivability, good base stats, and having at least one buffer and one trap spotter (IE, one force, one robot). Time Attack is all about powering through stuff as fast as possible, which generally means HUcasts + Forces, with the Forces taking the HUcast scapes so they can die for TP refills. Rangers are… well, sadly mostly useful to give their mags to a Hunter for the ATA boost.
General Challenge-
Top- HUcast, FOmarl, FOnewearl
Mid- HUmar, HUcaseal, RAmar, FOnewm, RAcast
Bottom- FOmar, RAmarl, HUnewearl, RAcaseal
Pretty obvious top tier- HUcast has the best stats in the game for what he is, the FOmarl is the best ‘clutch’ force who spends most of her TP on buffs and heals since she isn’t at risk of insta-gibbing compared to the other forces, while the FOnewearl is the best ‘nova’ force in Challenge with the TP advantage and the boost on the basic-level techs, which are the only ones that can be counted on in every challenge.
The Mid tier is mostly classes that bring something to the table but not as much as the guys above them. HUcaseals are just worse HUcasts unless they get their hands on some mechguns, while HUmars are basically worse HUcasts that can’t see traps. RAmars and RAcasts can actually do some damage with their attacks (And RAmars are probably the only non-Force class in challenge that can feel safe with their odds on hitting something) while the FOnewm is basically a worse FOnewearl except for the last couple challenges. The bottom is about as obvious as the top- RAmarls and HUnewearls benifit in normal PSO due to being extremely good Jack of All Trade characters, which is a detriment in Challenge due to the lack of resources instead forcing people into the set roles. RAcaseals are RAcaseals (seriously, Sega screwed this class over in the transition from PSO ver. 1 to the rest of the eps) and FOmars just tend to squish on contact with adversity.
Time Attack-
Top- HUcast, HUcaseal, FOnewearl, FOnewm
Mid- FOmarl, FOmar
Bottom- HUmar, HUnewearl
Rangers- RAmar, RAmarl, RAcaseal, RAcast
Rangers are literally a waste of a slot in Time Attack runs unless you get some absurd luck with a shotgun or rifle drop and enough power materials to feed him to make it hurt, but those same power mats can more reliably be fed to a HUcast/HUcaseal with a mechgun to turn them into a meat grinder.
The choice between HUcasts/HUcaseals and FOnewearls/FOnewms depends on the challenge- sometimes the raw stats of the HUcast/FOnewearl make them more useful, sometimes it’s the HUcaseal’s higher ATA/FOnewm’s boosted upper-level techs that make them the better pick.
FOmar/FOmarl can sub for the newman forces in a pinch, but their more limited TP means they have to be a lot more conservative. HUmar and HUnewearl are basically crummy alternatives to the roboHunters thanks to lower general stats, especially the defensive ones- important since TA Hunters tend to take a lot of risks.