Football sometimes is like rock, scissor or paper. Or pokemon (water, fire and grass), if you want.
That said,
lets rank opponents from Barça point of view:
Liverpool rank S
Barça is to Liverpool the same as bulbasaur to charmander. A night in Anfield could be a nightmare.
Juventus rank S
Solid team + Ronaldo
They defeated Barça 3-0 in Turim 2 years ago. Barça squad is better, but they have CR now. They conciliate physicality(Chielini, Ronaldo,...) , goals(Ronaldo) and talent(Pjanic, Dybala). Their game seems static, but it is effective. Srong contender to the title.
City A
City performs very well in premier league, but has had bad nights in cl, similar to Barça. Their style suits Barça well, but this year could be different, who knows.
Tottenham B+
We beat them in group stages (solid performance away from home), but that means nothing, bc ko stage is different.
United B
I havent seen much games from them, but Solskaer effect have been liberating. They dont scare much, but precaution. I think they are same level of difficulty as Chelsea last year in 1/16.
Ajax B ( could be more)
To me, they played the most atractive football in the competition. Their squad isnt exactly City, so it will depend if crucial players are suspended or not. Little changes in teams with less resources have tremendous impact. Lets hope they continue strong.
Porto B
Brahimi, Marega and Danilo are back, but Barça should go through.