Now I want to replace one of my team with a Mega Sceptile. Which Pokemon should I replace in my team for the best competitive play?
As Squirty(5qwerty) already mentioned, you can only Mega Evolve once per battle so logical choice would be to remove Mega Alakazam and Mega Salamence if you really want Mega Sceptile. Mega Salamence is also considered an Uber Pokemon so you wouldn't be able to play OU with this team. If you're building for Ubers that's fine but your team is really lacking the fire power to deal with the monsters in the tier.
If you want to build a competitive team for OU, you need to reduce the amount of Dragons as Fairies and Weavile will annihilate your team. Jolteon isn't very strong and I would recommend switching it out (Mega Sceptile can also absorb Electric type moves while being faster) for something else.
In the future please include items/abilities/exact EVs/movesets in your post. You could even add a little explanation for each Pokemon as to why they are on the team though most of us can extrapolate that info from the sets they're running.