Celebi being weak to seven types makes it difficult to mind something which resists everything it's weak to... besides the
but one Pokemon which can deal well with the majority of Celebi's counters is Terrakion.
Everything which is super effective to Celebi Terrakion either resists or is neutral to, and it's the same for Terrakion's weaknesses and Celebi.
With that in mind, I'm going to suggest
Choice Scarf Terrakion
Terrakion @ Choice Scarf
Trait: Justified
EVs: 4HP / 252Atk / 252Spe
Jolly nature
-Close Combat
-Stone Edge
-X-Scissor
-Rock Slide
While Terrakion is not going to be able to switch in to Meloetta, as both Close Combat and Psychic are super-effective, neither can Meloetta switch in to Terakion... unless it especially wants to take an X-Scissor. Terrakion can also outspeed and revenge kill Meloetta-P with super-effective Close Combat.
Meloetta aside, Terrakion makes a great addition to the team. It can take most moves that would be dangerous to Celebi very well, as well as being able to remove Pokemon which wall Celebi, potentially allowing for a sweep.
Choice Scarf Terrakion is also a powerful revenge killer and so a valued addition to any team. It also doesn't really need any other Pokemon implicitly, so allows us to keep our options open, and is not the easiest Pokemon the counter.
EDIT:
let's have our pokemon have at least decent synergy with Meloetta too:
The pokemon we are suggesting are for
Team 2 (to pair with Celebi, not to counter it!)
The pattern for choosing pokes will be 1 2 2 1 1 2 2 1 1 2 2 1
Edit2:
Xelacelle
I want Genesect next so I don't want a fire weakness from Celebi.
Who is that? xD
Also, I'm thinking Genesect may work pretty well with Meloetta. We'll have to see what we get first though I guess.