Well, I tried several systems.
My own
Dark Fantasy Basic (BX retroclone) says this (note: you crit if you beat AC by 10 or more):
Critical Hits and Fumbles
A critical hit in combat allows you to add your level to
damage. A natural 20, whether it is a critical hit or not,
allows another attack immediately, against the same
creature or another if you're using a melee weapon. This
second attack is made with a +2 bonus if you make it with
a different weapon (off-hand weapon, shield, kick, etc.).
[...]
Under usual circumstances, there are no fumbles when
making an attack. PCs are competent enough to avoid
looking stupid in every fight.
HOWEVER, I tried
Rolemaster and IIRC there was a table for everything. I really, really disliked it - I think such tables should be used sparingly - but some players really liked it.
Another idea that I hate is the
"confirming crits" nonsense. Even PF is avoiding this for their new edition (and PF 2 is also using the same formula I used for crits
).
Equally silly is the
GURPS idea that you've gort a critical hit table... hat does nothing half the time!