I find that most first books in a series works as a stand-alone, but once the publisher has confirmed that there will be a second/third to come, the next book almost always ends on some kind of cliff-hanger.
If it does I will still pitch the book in the recycler, and the first of the series as well. I will still forever ban your books in my personal library.
I think you should definitely mention that it has series potential, especially with fantasy or sci-fi.
I disagree. If this potential is true, it will be immediately obvious to any savvy agent and your publisher, and you will have wasted a few precious words in a query which must be as compact and effective as a haiku. It also sets up expectations you may not be able to fulfill, leaving a sour taste in the mouth of the agent and publisher.
One possible way to handle the series-potential idea is in the part of the query we typically put near the end.
"I am seeking representation for My Perfect Book and possible sequels, prequels, and tie-ins. I am also interested in expanding into young adult and techno-thrillers."
This sets up no possibly hard-to-pull-off expectations, but does show that you have thought deeply about your profession and its future. Agents and publishers typically want to work with writers who will produce lots of books in the years to come.