Tank Leafeon is outclassed by Tangrowth. BP Leafeon is outclassed by Celebi. Sweeper Leafeon is outclassed by Sceptile.
Period.
Compariong Sceptile to Leafeon is a mistake IMO. They don't share any roles.
Now BP Leafeon vs Celebi is an interesting case. I assume you're talking about a BP Swords Dance Leafeon vs BP Swords Dance Celebi. (Leafeon can provide wish support as well, and if you want a Swords Dance Baton Passer with Wish support, Celebi is out of the picture)
So looking at a Swords Dance + BP + attack + filer set... Celebi has better special defenses than Leafeon. Because Celebi has much higher HP, it will also take physical hits quite well. Leafeon only bearly beats it. Having 70 less hp than Celebi at lvl 100 really hurts it.
On the other hand, Celebi does not seem to have any STAB
physical grass attacks. Plenty of special attacks, but none are physical.
So I would see Celebi as the special swords dance baton passer, while Leafeon as the physical swords dance baton passer.
Anyway, it just looks like Celebi and Leafeon are just too different to group them together. It is fair to compare tank-set Leafeon to tank-set Tangrowth, but Celebi and Sceptile are just too much of a stretch to compare.
Now i was planning a team and leafon was actually going to appear on this team, but it didnt and only missing one move stopped it from this. if they had given leafon aromatherapy it would have completed him for me. with SD, BP, leaf blade and aroma therapy, you could pass, attack, or just be a cleric with some good defense. sadly this is not the case.
I'd say Wish support is a cleric role. It ain't aromatherapy/heal bell, but it gets the job done in a different way :-p