After revisiting the game after a long slant of Left 4 Dead 2, I decided to rework my version of the tier list. Based upon usage throughout the game (1-27th for me)
Top - Extremely useful from the moment you get them
High - Useful a majority of the time
Mid - Useful in certain situations/with a very specific strategy
Low - Not very useful outside of a few specific situations
Bottom - Almost never more useful than another character in the same situation
No specific order within tiers
TopMeiling - best tank evar
Reimu - of course
Iku - yes
Nitori - uhhh
Alice - durable, damaging, highly flexible
Mystia - see Alice, with less damage and durable but more speed and flexible
Yukari - Spiriting Away and IN Quadruple Barrier helps supplement/relieves Reimu's defensive buffing needs
HighMinoriko - and that's after I've spent about 100ish levels in SP
Marisa - a great random sweeper and Master Spark can be used to supplement your offense
Flandre - dies to magic, but inflicts massive damage with Starbow Break/Levantine (I cannot spell the thing correctly, so I'll stick with the Nanoha spelling). Would be top if it weren't for the composite spells' massive drawbacks
Rinnosuke - Remilia on drugs
Remilia - probably is your best attacker up until recruiting Iku, and will still be one of your best attackers for a long time, while being durable
Patchouli - when stuff needs to die
Shikieiki - when stuff needs to really die
Kaguya - Hourai Barrage and Stone Bowl when Hourai Barrage doesn't work. Almost as good as Iku at sponging magic
Reisen - tremendous help throughout many debuff and fire vulnerable bosses
Chen - great bosskiller
Aya - Chen 2.0 if buffed, very good supporting attacker if not
Kanako - great cold and wind nukes
Suwako - one of the few Mystic DPSers and the strongest Nature attack in the game. Awesome paralysis
Yuugi - KOi3S is awesome, really durable for such a good attacker
Suika - essentially Yuugi, for me
Orin - Needle Hill and Blazing Wheels really does pwn
Renko - Debilitate and Galaxy Stop are broken, if the target(s) are reasonably vulnerable to debuffing and/or paralysis
Tenshi - most stuff bounces off of her
Komachi - laughs at boss!Eiki, Rasetsu Fist, and a few other super-dangerous attacks, is a good Spirit DPSer and source of disabling
Sakuya *PURE ATTACK* - great DPSer with great durability
Ran *PURE MAGIC* - see Sakuya
Mid Wriggle - durable poison/decent nature damage when buffed
Ran *DEFENSIVE* - Great DPSer or buffer, you can't have both sadly
Keine - Great random encounter tank and buffer for bosses
Yuuka - Master Spark is a great anti-random emergency tool
Youmu - would be high if it weren't for her severe SP troubles for the longest time
Yuyuko - same, except she's slightly less affected by it. Slightly.
Utsuho - lacks a major-power nuke, but Giga Flare and Hell's Tokamak are strong enough, joins really late for her effectiveness though (usually last/second to last to be recruited)
Rumia - Moonlight Ray freaking hurts
Sanae - based on Miracle Fruit spam. As a pure MAG healer, I have not tested
Cirno - best random disabler until Mystia comes along.
LowMokou - poor man's Alice, really. Shame, because she has a lot of potential otherwise
Sakuya *DEFENSIVE* - After a certain point (basically when everyone reaches about 200 speed), speed buffing is nowhere near as awesome as it once was, and Aya/Sanae do a better job (allows you to manipulate party speed better)
Maribel - poor man's Reisen, except with better element spread, worse buffs, and much less usefulness time (Reisen can be used against the Great Stamp, Yukari, the v.2 bosses, the boss rush v.1, and the final bosses before Mari even comes into play). Outside of Destroying Quadruple Barrier and a very situational Liberated Abilities (fully buffed Mari vs. a Mystic-weak, low defense, low mind, debuff vulnerable target), whatever she can do, Reisen does better, on top of coming earlier
BottomEirin - really only shines against Eiki as Komachi's healer, and in reality, she needs Minoriko and/or Sanae as a backup in order to get Komachi back into running shape after a Last Judgment or other super-damaging attack. Minoriko generally does enough healing for Komachi for the rest of the game. Minoriko's also a better attacker against one specific boss (Yukari), and all of your other healers can do better when they don't need to heal (Sanae, Reimu and Minoriko's buffs are more useful, Rumia, possibly Sanae, and possibly Reimu can put up better damage), useful on paper, extremely situational in practice
Edit: Eek, forgot Sakuya, and also split Ran and Sakuya into two major schools that people generally evenly divide them into.
