So far, I've noticed Divine Excess is far and away the strongest circle in the game. However, this is absolutely terrible, since it won't let me level up more than two characters at a time, only having two characters level up at once is a bit of a pain, and seriously, it's like IDDQD. I wanna have some fun, use some abilities, and remember to step on heart shaped traps every once in a while.
I'd like to avoid fatemate and no-heal. Awaken isn't TOO offensive, but I'll gladly take a weaker version of the same circle without awaken.
What circles do you guys use?
Do people actually read these words?
As far as I know, some of the strongest and/or most favored Magic Circles are...
Divine Excess: its stat growths are amazing, but I personally hate it for the fact that you only get one Sub Character, making dungeon-delving extremely hazardous, and you can't even heal damage dealt to you. While this Magic Circle has the potential for the highest stats in the game I don't think it's worth the downsides, especially when you get into the postgame and in a 99-floor dungeon run even a single screwup near an Earthquake Trap with this one can mean you failed. If it works, it works REALLY well. If it doesn't work, then you're basically dead.
Water Lily: I found this Magic Circle to be extremely useful for the early postgame, when you're in need of a decently balanced Magic Circle that gives you a ton of Mana to throw around and you don't have the best artifacts. Later on it's usually handily outclassed due to the absolutely useless rear position and its complete lack of any Critical/Speed spaces, but if you're not too far into the postgame it's worth a look into.
True Champ: One of the few Magic Circles with a gimmicky Fatemate/HP-%/Defense combination that actually works. It's a bit risky due to how easily it can fall apart, but with good enough artifacts it can lend itself pretty easily to gargantuan stat buffs and it's favored by many for its decent Sub setup and its high stat-boosting rate. Raising Defense is a bit difficult, though.
Kaiser: It's incredibly boring and it doesn't have a huge amount of spaces, but it has all of the essentials and offers enough potential stat boosts to be worth looking into. Five Sub slots means that it has good HP and a decent amount of room for mistakes, but all of the best areas are positioned on the sides and you can take a beating fairly easily.
Fuma Kotaro: This is an Awakening Magic Circle, but it's useful for the massive Widen Awakening chain and the two characters at the back splitting all damage between them. Don't cast away Awakening MCs too soon, because Awakening stats don't actually affect the monster level in Tri-Geons, allowing you to have a sort of trump card for if the level accidentally raises a bit too high near the end.
Ultra M: Unlocked by clearing a Neo-geon or Tri-Geon with Immortals chasing you turned on. It requires an insane amount of time and effort to get to work, but after you do so it's a fairly decent MC that offers eight Sub Characters with no strings attached like Fatemate or HP -%, a rarity. The one stat-boosting area it does have is one of the best candidates for a Widen chain in the game. Careful, because it has no Ability Spaces. Probably not recommended, but it's basically an Awakening-less Fuma Kotaro without ability spaces and an enormous more amount of effort required to get it to work.
Lord of Heaven: One of the other good special Magic Circles that you could possibly unlock in your lifetime, you get it by clearing the Tri-geon and finishing on Heaven's 99th floor, Divine Army Area. It's actually pretty good - it doesn't have a huge amount of potential for stat growth, but all damage is split between four Sub Characters with a maximum of 799 HP (HP -20%), while the other four slots are dedicated to either abilities or stat buffs. Unbelievably stable in its Sub Character configuration, but unless you have some amazing artifacts it probably won't buff your stats too high.
Honorable mentions include Ultimate Form (Dragoon MC 20), God Exceeder (which you will never get in your lifetime; clear either Neo-Geon or Trigeon at level 9999, forgot which), Core and Megalomania (Guardian MC 20 and 19, respectively; has eight Sub Character slots but a lot of defensive penalties and some spaces are nigh-useless), King of Gaea (Ranger MC 20), and depending on who you ask includes a few other lower-level MCs. A lot of the others, like Dark (Shaman MC 20) and Dark Child (Swordmage MC 20), are underpowered, laden with defensive penalties, and aren't worth shooting for.
Bumping for great justice.
I'd also recommend you avoid the following Magic Circles:
Divine Excess: Great Magic Circle for boosting stats sky-high, but it's perhaps the worst defensive endgame Magic Circle there is and as far as multi-purpose uses go, as you go farther and farther into the postgame it becomes a piece of junk due to only having 1 Sub with Won't Heal. Good for endgame and early postgame, useless for all else. I seriously have no idea why it's so popular.
Dark Child: Awkward Won't Heal setup, elemental resistance drops, and not a huge amount of stat boosts either. It's okay, I guess, but... it's more like an 18th Magic Circle than a 20th. Really, Swordmage doesn't get much in the way of good Magic Circles. Avoid.
Dark: Shaman's ultimate Magic Circle; fails due to having abysmal stat gains, having other Magic Circles which outrank it as far as spell boosts, and having really awkward setups along with a ton of Awakening spaces that neuter its combat potential even more. This may actually be the worst ultimate Magic Circle.
King of Gaea: Suffers from mediocrity, really - nothing really big you can do with the Magic Circle, so unless you absolutely cannot live without Gate Revive, there are WAY better options.
Core: Personally I can't stand this one for purposes other than training, due to the mass amount of HP -% spaces and the fact that a good half of the Sub Characters can't do anything to make good use of their Mana. It also suffers from the same mediocre stats that most of the others on this list have.
Any special MCs not named Lord of Heaven, Ultra M, God Exceeder, Lucky 7, or possibly a few others: Avoid. Most special Magic Circles are gimmicky and/or wholly mediocre, though a few of them can work. Avoid Pass The Summit in particular, that one's nightmare-worthy.
Any MCs below level 17, sans Crazy for stat grinding: This one should be obvious.
And someone please let me know if they've had major successes with any of these, I may need to re-evaluate my opinion on them in that case and possibly try them out more myself.