I was asked what kind of ponds I'd had over my career in gardening and what I'd recommend.
To begin - I've dug my own natural bottom pond (well, I didn't dig it but I had a machine do so - it was approx 150x45' and ranged in depth from 8-16'). I've had plastic lined with no rocks - plastic lined with rocks, small preformed plastic liners (mostly in the greenhouse for displays but two did wind up in one garden, plastic deck ponds and wooden barrel deck ponds.
These were all constructed by myself (if you don't count the hi-hoe that dug the farm pond). :-)
What systems do I recommend? It depends.
What is it that you want a pond to do? Do you want to do the work yourself? How big is it? Is it a fish pond or a plant pond or both?
There are way too many questions to answer to give you one definitive answer.
Do I like Aquascapes? (the company in the previous posting) Sure. They provide a turnkey system for folks that don't want to build - can't build and haven't a foggy about how to build. They waltz in and give you a proven system that works from the get-go. They provide instructions on care and support system. You will pay for this. (As an aside - I do believe that Aquascapes has designed a system that works well for almost any situation if the pond owner follows the operating rules - as a landscaping "system" it is to be admired.)
But you will also pay for any landscaper to install any system. Some landscapers know what they're doing and some don't when it comes to ponds. Fact of life. And unless you can get references, you're the one taking the chance. So get references.
What system would I use when I build my ponds here? (I just bought this place last December and have been building gardens here ever since - but there's only so much time, energy and money to build so much every year on an old house that needs work as well) I want to emphasize the ponds have been designed so it's not "if" I'm putting them in but "when".
I wouldn't use Aquascapes or any other company. I can build my own. I'll figure out filter and pump sizes and layouts all by myself. I'll look for supplies at good pond supply places and install the system myself (the current design calls for filtration by bog - small water falls - and rather large water plant installations).
IMHO - you wouldn't spend that much money (a good pond can be expensive) on anything else in your life without doing your homework. I wrote an entire website on water gardening to give folks this kind of information.
So I dont' recommend any particular supplier or system because it depends on what you're going to do with your pond. I do recommend high end supplies (you'll pay for poor ones in other ways - like cheap pipe that cracks in the winter freezing and leaks so you have to dig up the landscaping to replace it with the expensive stuff you should have started with) ;-) and not taking shortcuts in construction techniques (you'll pay for these as well).
Am I an expert? Nah. I'm just a guy who's been there - dug that pond.