And why would we be asking this question? Much depends on the answer, as will become plain ...
Firsr off, meter bases themselves come in two varieties: with and without sealing 'rings.' The ringless bases have the entire front cover slip over the meter and hold it in place. Both types of bases use the same meters. PoCo specs determinewhat is acceptable in your area.
While still quite common, the simple round meter base is obsolete. It has but a 60-amp rating and lacks wire space for any sort of splicing; the practice was to selectively 'skin' portions of the conductors when placing more then one meter.
Now, were you asking about the pattern of the lugs, or 'jaws?'
The most common is the single-phase 4-jaw meter. More recent versions have one of the jaws at a right angle to the others, to prevent the meter from being 'reversed.'
Three-phase locations require a 5-jaw meter.