just wondering how people would rank and compare these firms if you were to start off after graduation.

It does of course depend upon what areas you are interested in and what your role would be but...They are broadly similiar in terms of perception, being below MBB and above just about everyone else. This will vary from country to country, for example you could include Roland Berger as they are so big in Germany. Some would include Mercer in its various forms too. They are top ten but not top 3. If pushed I would say ATK and Monitor are more highly regarded.

How does Oliver Wyman feature here? It is very big after the merger. Is LEK the best of the bunch if intention is to go into private equity or hedge funds later on?

LEK -> due diligence / private equity heavyATK -> focus on operations, especially supply chain managementOC&C -> is a sort of mix between the two in terms of work done: a lot of due diligence, but relatively heavy on operations projects for smaller companiesMonitor -> arguable, the most prestigious of all, working predominantly for large multinationals on strategy projects