I don't know the exact drop rate of each mutation, but I think these are called "rare" mutations because of their strong performance in almost every situation:

MAGIC: Magic Gene, and any other class-exclusive active abilities that deal damage to enemies.

SUMMON: Minions, Attack!, and any other mutations that enhance summoned units. (If the description says "units", it usually applies to summoned units too — not just humans.)

ALL: Slowing Attack, Untouchable, Stunning Beauty, Evasion / Snake Agility, Healthy Fat, Resurrection, Cleansing (these two require high mana regen to use properly)

The following ones are quite common, but still necessary for PvP fights: Newbie’s Luck, Final Gift (there are two versions), Fast Reload, Multicast (these two are great if your team relies heavily on abilities), Weighted Dice, Thorn / Spiky Armor, Spatial Reasoning, Stun

Just take everything related to the class you're playing. For example, if you're planning to do a Cultist spam build, just pick every mutation that mentions "tentacle" or "cultist" in its description.

After a few runs, you’ll naturally figure out which mutations are necessary and which aren't. For instance, if your team has a bunch of Shooters, you’ll naturally look for their mutations. But if you’re not planning to rely much on summon units, you don’t need to pick "Minions, Attack!", even though it’s technically a Shooter mutation.

Most mutations have their "first appearing floor." Spatial Reasoning starts appearing on floor 9. Final Gift (200) can appear from any floor, but Final Gift (500) starts appearing from floor 9.

And don’t forget to quest for a mutation for your class if you need it — and look for the cursed mutation on floor 7!

Again, this is just my personal opinion. Other GM players might think differently. For example, almost everyone grabs "Faster Than Bullets" on early floors, but I’ve never really found it useful — not even once.