I was asked which auxlang would be best to concentrate on: Ido, Esperanto, Interlingua, Interlingue, or Volapük. Note: As explained below, Ido is now my second choice, Interlingua my third...
Esperanto is by far the most commonly used, and also has the greatest number of resources both on the Web and off. Many countries around the world have their own local organizations from which you can obtain books and magazines. It also has the most regular grammar. The only real disadvantage for the beginner is that its vocabulary contains a greater proportion of Germanic and Slavic roots than the others do; even though *I* like that, it may make the vocabulary a bit more difficult for people who are more familiar with Romance languages.
Second best would probably be IALA Interlingua. Fewer people use it, there are only a few organizations; some books are available. But it does have a good presence on the Web, and it is fairly easy to read for somebody who knows a Romance language, because the vocabulary consists of words as they most commonly appear in the Romance languages. But its grammar is much more complex than Esperanto's because it tries to embrace the grammars of all the Romance languages and retains many of their irregularities, so it is harder to write.
(A couple of years later, I'm wavering... tempted to swap Ido and Interlingua. If the Net is a good indicator, Ido appears to be used far more than Interlingua. And knowledge of Ido would give you a good hook into that dominant auxlang, Esperanto. I'm really rather surprised that Interlingua isn't enjoying greater popularity. It's a decent enough language, albeit a bit more complicated than necessary. But there are relatively few Interlingua lists and websites, and I'm suspicious of many of them: Most of the sites not consisting of dictionaries or grammars seem to be very small sites owned by very pretty girls! (A post on an Esperanto list once jokingly suggested that perhaps pretty girls could be used to lure students to Esperanto. Hmm. Not that there aren't more than a few pretty Esperantists out there too, but the Interlingua babe ratio just seems way too high. (Most of those sites were on Geocities so disappeared later.)))
Ido would be my third choice. It's like Esperanto with some Interlingua mixed in. The vocabulary is basically like Esperanto with a touch of Interlingua, and the grammar is like Esperanto's but somewhat simpler (maybe a little too much simpler, but I can't really say for certain because I haven't used it enough), and much simpler than Interlingua's grammar. Esperantists can read Ido and Idists can read Esperanto without too much trouble. I like its simplified grammar and some of its Romance vocabulary but I think it might have had a better chance if the authors hadn't replaced so *much* of the Esperanto vocabulary with Romance roots. Ido could have been my first or second choice, but it's not used by as many people as the others, and there aren't as many resources available.
Interlingue (also known as Occidental and Auli) would be my fourth choice. It is *very* much like Interlingua. Practically speaking, either can be described as the other with some of its vocabulary replaced by an alternative root from the same or another Romance language, [with Interlingue containing some Germanic influence in both vocabulary and grammar,] some of the words spelled just a little bit differently, and some of the grammatical suffixes changed slightly. People who can read one of them can usually read the other. I put Interlingue in fourth place because fewer people use it. Also, but this is just a personal preference, because when I compare Interlingua and Interlingue words that differ, I find that I usually prefer the Interlingua choice [when both draw from Romance vocabulary].
Though I find Volapük interesting, I think that very few people know it. I've only found a few websites for it, no books or magazines available anywhere, and only one person who was actually writing in it.
Lingua Franca Nova looks interesting. Like Interlingua, but without the unnecessary overcomplication. And you can read La aventuras de Alisia en la pais de mervelias as a real paper book or pdf. Still a small community, but a nicely designed language.
(a) signifas anglan influon, (e) signifas esperantan, (ĝ) signifas ĝermanan, (s) signifas slavan, (sk) signifas skandinavian.