It depends on which country you think most of the participants will come from.
If it's an international conference, bear in mind that US / Canadian visas - even for short visits - are NOT easy to get for people from many countries. Even if it's for the purpose of a conference, I have heard many anecdotal stories from friends and colleagues who require a visa for US / Canada to have their visa denied. Of course, if most of the attendees are American then it makes sense to have in the US.
If most of the attendees are expected to be European, anywhere within UK / EU would be a good choice. Costs of organising an event in UK / EU would be higher too though, as well as accommodation costs, so this may be a limiting factor.
Considering ease of getting a visa, travel costs (flights, etc), accommodation / venues, I would highly recommend either Hong Kong or Singapore. The reason is that being based in Asia, the costs for attendees to stay or even for organisers is lower than that in America or Europe. More importantly, people from most Asian countries as well as Europe / North America / South America can easily get visas either on arrival or without much hassle. Both Hong Kong and Singapore are major aviation hubs and hence flight costs are competitive too, especially for attendees from Asia. There's a wide choice of hotels for venues / accommodation to cater to any budget and both countries have excellent telecommunications infrastructure (Internet and mobile broadband speeds are great) and public transport.
So it really depends on who's attending, but on a purely global scale for a global conference with many attendees, somewhere in Asia with lax visa rules (this rules out places like India, for instance, since nearly everyone needs to apply for a visa for India) like Hong Kong or Singapore would be a good choice.
