Hmm... Well, I'm not sure if you'd want spoilers or not, but Ill try to give suggestions in a spoiler-free manner.
The first thing is to try and make a balanced team. With that in mind...
Charizard (Fire/Flying)
Lucario (Fight/Steel) or Steelix (Rock/Steel)
Roselia (Grass/Poison) (might need to consider evolving to Roserade for extra stats)
Lapras (Water/Ice) or Vaporeon (Water)
Snorlax (Normal) or Kangaskhan (Normal)
Now then, in making a balanced team is important to consider the advantages and disadvantages of the pokemon. Because of that I would choose Charizard over Delphox since he's less vulnerable. I'm sure that you will consider mega-evolving or not depending on who you're facing.
This one I plan to leave it to you. I would take Lucario, but I believe Steelix has one less weakness than Lucario, so Steelix might be the better choice.
Is funny that I don't know much about grass type, especially considering that Grass starters have been my favorite in 3 of the 6 generations, so I don't much about Roselia. However, considering that we're trying to make a balanced team, I think Roselia is a better choice that Butterfree simply because it has better stats.
Now then, in the water/ice department, you have three pokemon at your disposal. I would choose Lapras because of the bigger overall stats, but the other two aren't too bad either. Vaporeon has less weaknesses I believe, so he could be a good choice too. I don't know about Aurorus, but if you go with Steelix over Lucario, then I would not recommend Aurorus since he's also has a 4x fight weakness with his dual type and is rock type too, like Steelix. and you already have two pokemon weak to fighting in a steel and a normal pokemon in your team.
Now, Kangaskhan and Snorlax and two pokemon I don't much about either. However, Even though Snorlax has better stats than Kangaskhan, Kangaskhan is faster and can Mega-Evolve. So if you want someone faster, Kangaskhan is your choice. But if you want more defense, Snorlax could do good.
Now, the final space is open for the taking. I was wondering if you could add one more pokemon from your team, but I can't really find one. So my advice would be to et another 6th pokemon. And if I can give an advice, I would advise you either a Dragon type or an Electric Type. If you plan to Mega-Evolve Charizard during the Elite four, then I would suggest an electric type that is not Ampharos. Ampharos mega-evo is also Dragon type, so that would not help. If you wish, I could plan with you so that I can trade you an Electrike. Manectric might be a nice addition to your team, plus he can mega-evolve too. Another good choice is Garchomp but again, two dragons might not be good unless you may not want to mega-evolve charizard.
Oh, and one more thing: If your pokemon are EV-trained (I'm sure you confirmed this earlier), Try to teach sweet scent and petal blizzard to Roselia temporarily. Floatzel have a decent horde spawn rate in the victory road and they give a very good about of experience. Petal Blizzard is a move that hit all pokemon if memory serves, so you can take them down fairly quickly.
Anyway, I think that's all I have to give for now. Not sure if this is a good advice, but I hope that it is of some help.