Strong Common

Particularly powerful "common" cards (although some are not always easy to find), these are quite nice for improving the deck of a player starting out, as they only have 20 levels (and 30 for EX), so will require less sacrificed cards (and thus less energy spent gaining low-power commons). Some PM commons have attack and defense stats that rival rares, and if you're really lucky you'll get one with high stats AND a skill to use in battles.

They are also exceptionally good for enhancing high-level cards later on, as the amount of XP gained from a same-color EX with a PWR requirement of 10 is very high.