Visa-friendly country for an international conference?

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.

European countries tend to be the easiest for most nations to get to, when you consider transport costs and visas, as short term EU visas are relatively simple.

We don't have the restrictive policies the US have, for example.

London prides itself on being an ideal location for conferences, as Heathrow Airport has connections to everywhere but the Netherlands and Germany are also very well connected.

Like my answer on a related question, the Maldives would be a good candidate. Its international airport seems to be well connected. Citing Wikivoyage:

The Maldives have a remarkably easy visa policy: Everybody gets a free 30-day visa on arrival, provided that they have a valid travel document, a ticket out and proof of sufficient funds, defined as either a confirmed reservation in any resort or US$25/day in cash.

Personally I don't think the visa fees will be a deal breaker. I know from experience that getting visa for the Netherlands for conference attendance will be easy. Consulates do check visa applications for conference and if attendance is confirmed I have the impression that visa are handed out easily.

The problem will be more the cost of living for accommodation and transport from and to the airport.

You could choose to use the Big mac index to identify the impact the average cost of living will have on the attendees from a less wealthy country.

If funding is no issue, I would go for the Netherlands. That country is well connected, has one of the fastest internet connections, and if you don't mind actively communicating with the different Dutch consulates there shouldn't be that much issues regarding visa. You need to do this because there are always "participants" using conferences as a way to get into a country, but with no intention to actually participate.

Dubai has become a regional hub for international conferences, and it seems to tick every box as far as conference must-haves for your international audience:

1. Easy visa requirements. Most nationals can get visa on arrival; others can get visas through their hotel or airline at minimal cost.

2. Great transportation links. Emirates has one of the largest networks; plus Dubai is a hub for Qantas. There are direct flights from almost every corner of the world.

3. The city itself has a well thought out transportation system (including the newly introduced Dubai Tram).

4. Hotels. Don't need to say much here - everything is within reach as most hotels are along the main Sheikh Zayed Road.

The problems with Dubai (same as with Singapore) is the distance if the majority of your attendees are coming from beyond Africa.