ProjectEnd
Superstar
It's difficult to say 'best' as that evaluation is subjective, but I've long advocated for aA as the City's superior firm. When taken as an oeuvre, there are only really two 'bad' buildings in aA's portfolio: Form and Alter, both of which, as @condovo rightly pointed out, are for the same aesthetically-challenged client. There are more singular buildings in HPA's portfolio that I would say didn't work out: 2 St. Thomas, Ivory, Grand Palace, The Code, Novus, etc."Exceptional" is a little much. aA has done a great job all things considered, but there was a time where they literally rehashed the same designs for every project. Yea, the developers were probably asking them for it but I don't think that warrants an "exceptional" grade. No doubt they influenced many firms over the years so good on them. I'd say lately they have done more interesting work. Still a premiere firm. I think HP has surpassed them.
Another issue with a direct comparison is that HPA is a far more typologically diverse office - I'm sure Peter et al would love to do all sorts of buildings, but since Adrian passed away, pretty much all of their (very good) institutional work seems to have dried up (Brock's Cairns Complex is the only one that comes to mind).
Trust me, between the two of them, for firms of their size, they're either first or second, depending on who you like more. Given the proclivity of different client groups, I just happen to think aA is the more consistently superior office.