Ground Rules

Score in Might & Magic is measured as 1000 x Fame / Time, where time is the number of days completed. With respect to score, a new day starts at 9:00 am. Since Fame is calculated as the sum of the experience (kills and completed quests) of all four characters, which can be influenced by other factors than merely good gameplay, a few groundrules are necessary to make scores compatible. The following "cheats" are out.

[1] Respawning (Rockham, Silver Helm Outpost, The Hive)
[2] Reanimating (Dark spell)
[3] Repeated kills (by paralysis, or if there's no room for the body to drop down)
[4] The learning skill (another bug, it should affect experience but not fame)
[5] Hiring experience-boosting NPC's (same deal)
[6] Experience for money from one Enrothian well (especially when combined with:)
[7] Infinite money from the NWC dungeon

I do also rule out any other gains from the NWC dungeon, such as maximum stats and lots of potions, all of which would save a lot of time.

Let's see, how high scores can become if the ground rules are followed.
The following table shows how much fame is involved with the quests in Might & Magic VI.

Quest From Fame
Kriegspire 400
Baa MembershipAlmighty Head of Baa200
Jeweled EggEmil Lime200
Blackshire 520
Pearl of PutrescenceGhost of Balthazar20
Pearl of PurityGhost of Balthazar40
Werewolf's AltarMaria Trepan80
EmmanuelJoanna Cravitz80
Frozen Highlands 720
Free Haven Temple (Priest)Anthony Stone60
Sacred Chalice (High Priest)Anthony Stone120
Dragon Tower Keys (Battle Mage)Erik Von Stromgard60
Dragon Towers (Warrior Mage)Erik Von Stromgard160
Prince of ThievesAnthony Stone120
WinterErik Von Stromgard200
Free Haven 4320
Sherell IvanavehCarlo Tormini40
MordredZoltan Phelps40
Ethric's SkullGabriel Cartman60
Nomination (Cavalier)Osric Temper60
Warlord (Champion)Osric Temper160
Devil's OutpostOsric Temper160
Memory Crystal AlphaOracle of Enroth400
Memory Crystal BetaOracle of Enroth400
Memory Crystal DeltaOracle of Enroth400
Memory Crystal EpsilonOracle of Enroth400
Control CubeMelian2000
Arcane MagicMelian200
Mire of the Damned 280
Snergle's AxeAvinryl Smythers80
Book of LichesTerry Ros200
Silver Cove (220+) 200
AnkhLoretta Fleise40
MonolithEleanor Vanderbilt60
Sun Ceremony (Great Druid)Loretta Fleise(60)
Moon Ceremony (Arch Druid)Loretta Fleise(160)
Price FixingLoretta Fleise100
Bootleg Bay 80
Evil CrystalWinston Schezar40
Unward doorsFire Lord40
Ironfist (30+) 392
Sixth Letter to Wilbur HumphreyFalagar12
HarpAndrew Bester40
Damsel in distress (Crusader)Wilbur Humphrey60
Longfang Witherhide (Hero)Wilbur Humphrey120
Lord KilburnWilbur Humphrey160
Cure for SilvertongueWilbur Humphrey-
Letter to CouncilWilbur Humphrey-
Third EyeNicolai-
Mist 440
Enemy ListCharles D'Sorpigal60
Fountain of Magic (Wizard)Albert Newton60
Crystal of Terrax (Arch Mage)Albert Newton120
Hourglass of TimeAlbert Newton200
New Sorpigal 133
Letter to Andover PotbelloFalagar1
AngelaViolet Dawson4
Goblinwatch CodeJanice8
CandelabraAndover Potbello8
Spider QueenBuford T. Allman12
Shadowguild ProofFrank Fairchild20
Sharry CarnegieFrank Fairchild40
Chime of HarmonyJanice40
Total of all quests (250+) 7485

Additional fame comes from killing foes, including any Arena fights, and even from killing friends. There are a few special inhabitants of Enroth whose demise does not yield any experience points.
The level of the party influences the strength of the opposition, and there is also a random element involved. In a fast game like Xena's party, all inhabitants of Enroth together bring in about 10,000 fame.
However, it is not necessary to kill all the monsters in order to complete the game. The fastest time to finish is, as you can see from Seoman's log, only 5 days, which leads to scores of over 1,000,000.

If the ground rules as stated above are not followed, there is almost no limit to how high scores can become.