Best is a moving target variable based exclusively on what you want. I would like to have a nice LR Baggs dual source on-board system thru a modeling foot switch handling my presets and out to a Fishstick. I have the Baggs on-board system but I'm direct into a Fender Acoustasonic 80 watt, which isn't a dull ride at all. It's just real heavy and the FX are on the panel. I usually only put in a touch of reverb, anyway, but we're talking flexibility here and not necessarily just me. I just had the amp disassembled and all the electrical stuff cleaned. Sounds great but it didn't remove any weight.
Whatever you get be sure you have good control over feedback because the typical Piezo p'ups will get cranky. What I do to minimize that is turn the onboard preamp up where it just begins to power the Piezo signal out from flat. Those little things are real sensitive so they don't need much gain going out. Then I turn the amp gain pretty high. Then I roll the master amp volume in until the guitar is just audible and adjust the Notch control to cancel the feedback. I move between the master volume and Notch control until the volume is where I want it with no feed back. This has so far proven to be a number match on the Volume and Notch settings where a Volume of 3 is a Notch of 3. It works that way on this Fender amp.
I also find that a low guitar preamp gain pushing the Piezo p'ups keeps them well below quack territory. That means the acoustic quality is better at low gain. This will probably be the best way of maintaining acoustic guitar sound quality in any set-up. It is with my other guitar mounting a K&K PWM thru a K&K preamp. That system is particularly sensitive to preamp gain.
Regarding guitar on-board systems, I prefer the Baggs, then K&K. Talking systems, not just transducers. I've use the B-Band, Fishman, above two, and factory systems from Seagull and Yamaha. Yamaha IMHO, makes the best factory systems and is at or above Baggs sound quality. B-Band isn't a slouch but I did have some reliability issues. They have gotten better according to the folks at the Guitar Factory here in Orlando. I think Fishman has come a long way in their mag p'ups. Their acoustic systems I found gimmicky when I had them. One that I have not used but looks interesting is the D-TAR Acoustic.
http://www.d-tar.com/
Last word: I've heard some systems being played and the sound isn't right at all. Turns out it's because the guitar and the amp are out of phase with each other. Ensure the amp you choose has the Phase control on the panel. Some don't.