There are some interesting groups here.
Group A:
A good group with a fight for first place between United and Shaktar. In theory Leverkusen and Sociedad are the weakest but some surprises can come up and both can claim some points to the "big ones" of the group, so any result is possible here.
Group B:
Free passes to Real and Juve for the next round. Galatasaray can put up a fight but 3rd place is inevitable. Copenhagen is screwed.
Group C:
PSG goes to the next round as well, but the other three can be an authentic headache for PSG. Benfica (my pride and joy) is a good candidate for the other spot, not a very strong team but can cause some damage. Olympiacos will be fighting with Benfica for 2nd and I guess a really good fight coming up. Anderlecht can surprise but it's the weakest there.
Group D:
City and Munich with a couple more free passes but they can slip in Russia thanks to russian weather. Plzen is not a great threat for any place than forth.
Group E:
Is the weakest group along side with Group G (but balanced). IMHO, anything can and will happen, outcomes are not foreseeable and it's 4-way fight to the next round.
Group F:
By far the best and the most balanced group. This one will very interesting with another 4-way fight. I'll be looking foward with this one. Anything can happen.
Group G:
As I said above is one of the weakest groups here and Zenit is a good candidate but Atletico is really strong and can bring a good fight. Porto is a bit of an outsider but can surely put a hand on the first two places. And then there's Wien. The debuters so they don't want to give a bad image and they will fight for a place in Europa League.
Group H:
Barça, Milan will perform a good show. Celtic will use surprise factor. Fly under the radar and then strike with everything they got, like they did last year. I don't forsee a great future for Ajax though