I am probably going to sound like an oxymoron here with my reply...
I have my personal favourite companies for different reasons. Different companies have their unique qualities and special X-factor, which makes them exclusive. Be it a contemporary company, or a classical company, each has their own style of moving, training and staging. In addition, it also depends which dancers are dancing in the casts, no dancers will dance the same role in the same style or same feel. In fact, not even the same dancer will dance the same role the same way in 2 different nights.
Especially contemporary dance company, it is difficult to rate which are the top 5, because contemporary repertoires are constantly evolving, and with the current use of advanced technology, staging and dance design, contemporary dance companies are constantly updating themselves with new ideas and new experience. Also, classical trained dancers, such as Sylvie Guillem, are going into contemporary sector, and contemporary choreographers are working with classical trained dancers, such as Wayne McGregor.
The bottom line is, each and everyone of us have different 'taste' in everything. The same goes with picking the 'best 5'. For all you know, what one feels are the top 5 on the list today, may not stay the same in one's opinion, one month later...
If there is a "TOP 5 BEST", does it mean that there is "BOTTOM 5 WORST"?
I hope not. I don't believe there is any 'worst/bad' companies, cos everyone is different in their own right. Its about mix-matching the companies to the audience's distinctive taste.
[told you, i was being an oxymoron