I didn't vote on this because I'm relatively new and still getting my bearings straight. Regardless, keenly scanning through the list of default Soul Breaks is something everyone should do at least once because it illuminates just how many bad bad bad default Soul Breaks are out there and I graded harshly against it, as well as without regard for Relics or RM or MC2s (for now). This meant my average was noticably below the ~3.0 I'm eyeballing on the big board since every Core automatically had a 1 alongside about five other non-Cores.
Anyway, some thoughts
Gilgamesh: Underrated (5) - My only 5 due to Tauntaliate + perfect marks beforehand. TGISunday because with Sabin's Relic and no Sabin MC1, I need him at L50 for the FFVI event coming up.
Cloud: Overrated (3.5) - I don't have Organics or MC2 and aside from Retaliate (without Draw Fire), his skillset is far from unique. Also, Sonic Break is one of many of the terrible default Soul Breaks. Admittedly, having his MC2 would make him 4.5+.
Vivi: Properly Rated (4.5) - Vivi has perfect marks thanks to Focus Magic being T-Best default SB for a Black Mage. Also he got the extra 0.5 by having the highest base MAG (as did Lulu), but that won't count next time as Krile is introduced to narrow the gap, and MC2s start arriving for BLMs.
Terra: Overrated (3.5) - I graded harshly on default Soul Breaks. Also, compare her defensive stats to Ashe (FFXII), then check Ashe's Default Soul Break. Keep in mind that 3.5 is distinctly above-average - my distribution's average was slightly below 3.0 and mode was 2.5.
Others I have leveled to 50+ :
Bartz: Properly Rated (4) - Currently the only hero with Celerity 3, Spellblade 3, and Thief 4, This means he can run either Drain Strike/Tempo Flurry or Steal Power/Tempo Flurry at any time. Default SB has something besides damage (+50% DEF) but is useless versus Mage bosses and he's not drawing fire.
Celes: Underrated (4) - She's the essential Red Mage. A passable BLM with Devotion/Ruinga and defensively similar to Golbez. A capable fighter with Breaks/Spellblades and 130 ATK. Below-average DEF for frontlining but the highest base RES among all characters period. Passable "support" due to WHM3/BLM3 for non-Poison status debuffs.
Mog: Properly Rated (3.5) - The Bad Good WHM. I gave him MC1 credit and recognized his uniquely high defensive stats on a WHM4+ with that much MND, but he needs a Spear (or Dagger) to frontline and his default SB suuuuuuucks.
Selphie: Overrated (3) - Generic WHM4. Default SB has niche usage with Draw Fire I guess but I can't give any credit to it. I have her Morning Star which makes her a 4-4.5, but I deliberately avoided Relic bias otherwise I'd have Vivi at 5+.
Wakka: Overrated (2.5) - Trashy base stats, but practically every Support 5 has trashy base stats. That said if you don't have/need Magic Breakdown/Mental Breakdown/Full Break, there is a healthy supply of Support 3s (and a couple 4s) with much better base stats and more robust skillsets. Water Reels is a passable default SB since 1.25 Phys AoE/Water is comparable to Kick (3* Combat, 1.3 Phys AoE) and he can use the Evil Lantern. I'm probably not respecting Full Break/Magic Breakdown enough since I don't have either of them yet. (Again, this is why I didn't vote.)
Tyro: Overrated (2) - I'm assuming this is due to Sentinel's Grimoire. Otherwise, besides utilizing ranged Tauntaliate in the Scarlet Hair fight from last week he's a perpetual benchwarmer.
White Mage (1), Black Mage (1) - 'Do not grade with consideration for Record Materia.' Got it.