Which fan-subbing groups are the best? The worst?
Best at what? Visual quality of releases? Translation quality? General attention to detail? Speed? File compression? The last two and the first three tend to be in opposition to each other.
I'd say UTW are the least shitty speedsubbers out there right now. gg isn't nearly as bad as lots of people like to say, if you don't mind their tendency to troll with their releases (e.g., see their version of
Hidan no Aria, which was a parody of the whole "TL Note" thing and had subtitles in Japanese by the end, or their tendency to cut funny commercials in their releases). Mazui is quite good, but they rarely do more than one or two shows per season. Gotwoot isn't bad. And if no one else is subbing it, there's always Commie...
Coalgirls are said to be very good at the visual quality/attention to detail part, but they're abysmal at the speed/filesize part.
Coalgirls aren't fansubbers. They're an encoding group, and only worth using for archival purposes. They only do BDs and don't use their own subs; they take subs from other groups and re-typeset them.
Overall, I'd say HorribleSubs (AKA "We don't need no stinkin' QA!") are the worst group currently operating, as they occasionally turn out episodes with broken/missing subs . . . unless you need to maximize the amount of anime stored per GB of disk space, in which case their 480p encodes seem like a good bargain, so long as you check them and replace the occasional broken episode with something from a more reliable subber.
HorribleSubs isn't really a fansub group either. They don't do their own translation and subbing either. They just rip Crunchyroll/ ANN/ nico / Funimation streams and release those. That's also why their videos have such variability and lots of times aren't even in a consistent resolution — it's just taken from the best quality legal stream they can find. HorribleSubs doesn't even try.
I watched Ao No Exorcist in Horriblesubs and Coalgirls version, and that small Times New Roman-esque serif font Coalgirls used drove me up the wall (and messed with my crappy eyesight a lot). Is that a regular thing with them? If so I'll stick with a subbing group that uses sans-serif.
Lots of groups try to use whatever font they think matches the show best. Groups with more time (which Coalgirls have plenty of) will be more elaborate, while speedsub groups will go with what's easier.
Coalgirls usually has more than one subtitle stream. Since they only do BD releases and take subs from other groups, they usually include subs from multiple groups, so you have a choice which you want to use.
You'll also want to keep in mind where the encode comes from. If you watch a show on HorribleSubs, you're going to get a shit encode, because they just take it from the legal stream. Most speedsub groups get a TS from the Japanese television airing, and re-encode it using whatever settings they think is best. Lots of times, this also includes trying to improve the picture quality where possible (e.g., making darker shows like
Psycho-pass lighter so you can see more). Although archival groups like Coalgirls have high-quality encodes in terms of near-losslessness due to their ridiculous bitrates and always being from a Blu-ray, they're also kind of shitty in that they don't actually try to use encoding settings wisely.
You can always mux your own release if you want to mix-and-match subtitles and encodes. Just extract the .ass subtitles and mux them into the .mkv containing the video encode you want to watch.
Ji-hi used to be a nice fansub comparison page, but it closed recently. It looks like a
new one is opening up, though, which is good, 'cos I was missing a fansub comparison page this season.
ETA: And I just realized we haven't even touched on the heated topic of localization in subs...