What tier do you think they should be in?
---
Cleffa -> Clefairy -> Clefable
Tier: B or C
Availability: 0 badges
Stats: Decent
Movepool: Excellent
The problem with Cleffa is that it evolves through happiness and leveling up. While making Cleffa like you shouldn't be a big issue, Cleffa has problems holding its own in a battle. Magical Leaf helps Cleffa do some damage to her opponents, but it shouldn't be considered a viable, early-game offensive force.
Things get a little better when Cleffa becomes Clefairy. By level 34, Clefairy gets the Cosmic Power/Stored Power combo that can be helpful when set up. At level 46, Clefairy gets its first STAB move, Moonblast, which is helpful against Dark, Dragon and Fighting type Pokemon. However, the fact that one has to level up Clefairy to these levels is an issue itself. By the time it learns these moves, Clefairy is vastly outclassed by fully evolved Pokemon. Because of this, it is recommended that one evolves Clefairy after the player has taught it all the moves he/she wants it to learn.
Clefable is a useful addition to most teams with its abilities. Magic Guard is useful against Pokemon with Toxic, but Unaware is more useful long-term to prevent sweeps. Beyond the Cosmic Power/Stored Power combo, Clefable has access to a diverse movepool that can be used to make up for a team's shortcomings. Compared to other special attackers, however, Clefable lacks power and speed, both of which can be overcome with the CP/SP combo.
Overall, I would recommend that Clefable be placed in B or C tier.