I've been fortunate enough to have taken a considerable number of Backroads trips. Friends will therefore not infrequently ask me how to choose among the selection of trips Backroads offers. My first response is to choose one at random. Every Backroads trip is excellent!
But that answer usually isn't wholly satisfying to folks. So let me present some other criteria I've used to differentiate among trip options:
1. Two-Country Trips Are Especially Interesting
A considerable number of trips, especially in Europe, involve crossing international borders. Sometimes Backroads takes you across a border by van or bus; other times you can bicycle over the border yourself. This sort of thing is especially seamless in European Union countries without border controls. On the Slovenia & Italy bike trip I took in 2014, on Day 2, we took a quick morning bike ride into Slovenia, then returned to Italy for the rest of the day before heading back to Slovenia (more permanently) the next day. When you cross into a different country, it's like hitting a reset button on your vacation and you're learning (and eating!) anew. I've also found it sometimes to be the case that the second country is a pleasant surprise, i.e., you signed up for the trip to visit Country A, but Country B proves to be much more enjoyable than you imagined.
2. Take Advantage of Leaders' Language Skills
One of the many great things about Backroads leaders is that they are usually multi-lingual. This is especially valuable in countries where one is unlikely to speak the native language, e.g., Croatia, the Czech Republic, the Netherlands, Slovenia. Perhaps you might consider traveling on your own in a country where you speak the language. One might be less inclined to try that in a place where one doesn't speak the language. Let Backroads do the translating!
3. Consider Pre- and Post-Trip Arrangements
Backroads trips start and end in various places. Sometimes the trip starts or ends in a large city; sometimes in a small town. While, of course, Backroads will tell you how to get from a chosen small town to/from the nearest appropriate airport, one's life can be easier when the origin/destination location includes a large international airport, ideally with nonstop flights back to your home town. I find that I'm more tolerant of challenging logistics before the trip begins than after it ends; I want to get home as quickly as possible.
There is, however, one minor flaw with this list: perhaps (or at least among) the very best Backroads trips I've taken is the amazing Banff-to-Jasper Canadian Rockies trip. That trip is in a single, English-speaking country and it involves an extended van ride back south to Calgary (or east to Edmonton) after the trip is over. Yet it's a fabulous, signature Backroads trip. So maybe my initial answer is the right one: choose a trip at random; you'll have a great time!