It's difficult to describe in details the culture of each company in just one post. I think every company's culture description would require a separate post here on PL. But let's try:
BCG - has the biggest office in the region, in Dubai in particular. Very internation from the people and background point. It has good, but more formal culture.
Strategy& - is the second biggest firm in the ME and Dubai in particular. Has also very big office and very formal culture. The culture here sometimes perceived very strange, because it's so called "inshallah" - means extremely unreliable in terms of people and working culture, that promises will often not be capped. Strategy& is highly dominated by Lebanese people and less international, compared to other companies.
AT Kearny - has pretty good culture, but very different, heterogeneous, because every unit has its own, subculture. In general culture here is good, with many interesting people from different backgrounds.
McKinsey in Dubai is rather a boutique company. The office is not big in comparison to the previous companies. McKinsey culture is more relatively formal, but in ME it's more family-oriented because of the size of the office, where everybody still knows everyone.
Oliver Wymann - has friendly and healthy working climate. I really like their working culture.
Roland Berger - less than Oliver Wymann, but rapidly catching up (the company has doubled their size and presence in Dubai over the last 2 years, that is why they have recently moved to a new office last year). The culture is also very friendly. I really liked it, because I've spent roughly 2 years in Dubai with this company. It's dominated by Europeans and Lebanese with pretty international backgrounds and highly educated people.
Regarding a boutique firms which you've mentioned Frost &Sullivan is not considered a consulting company (in my opinion). It's rather a research company.
As for Alvarez & Marsal and AlixParners - they also have a good culture, but they are not as diverse as other companies.
So, if I were you, I would rather shoot for the top3 or top8 companies in the list.
If you need further advice on every company specifically, reach out to me.