Which hearing aids have the best directional microphones?

Technically, I believe all manufacturers use the same microphones but the algorithms that each digital hearing aid uses for processing vary greatly.  I think it would be difficult to answer exactly what "directional microphones are best in which hearing aids" because each manufacturer has their own proprietary hardware and software that is responsible for how the directional microphones operate/perform.  I believe the individual patient needs are best addressed in a personalized consultation with an audiologist who can review the communication needs and goals of the patient along with their level of hearing loss.  Recommendations for patients depend upon many variables and being guided by an experienced audiologist will be critical to the patient's ultimate success.  

Dr. Steele, and many others, have similiar answers to what I would respond.  I do think it's worth noting that it's important to have a full communication needs assessment to ensure directional microphones will be beneficial, as opposed to a remote microphone, or another assistive listening device (e.g. an FM system).  Also, the audiologist who fits the device needs to verify and valid the end-system (directional microphones, FM system, etc.) prior to the fitting, at the time of the fitting/on your ear, and after you have had an opportunity to use the device.

Timothy provides a good answer which I agree with, in that all manufacturers are typically using the same microphones, supplied by Knowles Electronics.  It's what the manufacturer does with the information collected by the microphones that matters, in that their noise reduction algorithms and strategies vary quite a bit (although some do copy each other).  As others have indicated, Phonak and Unitron have led in this area, Phonak in particular.  In the 1990s, when most manufacturers were touting their new digital technology, Phonak, which didn't come out with a digital model until the Claro in 2000, focused on directional microphones, and indepedent studies by researchers at universities, most notably Michael Valenta and Todd Rickets, indicated that it the directional mics that improve speech intelligibility in background noise, not the digital technology alone; Phonak's analog programmable aids at the time were outperforming the digital competitors mainly because of the directional technology.

Phonak continued to lead in this area, being the first to apply the digital technology to their directional microphone algorithms in ways the others hadn't.  With the introduction of Claro in 2000, it was the first application of digital technology in a way that didn't simply mimic analog systems, which is what other manufacturers had done.  It was the first instrument to have a system which could automatically activate, and its polar plot adapted to the background noise, so that if there was more noise on one side of you, it would alter the noise reduction to be more effective.

As other manufacturers struggles to keep up, copying Phonak's efforts, the Swiss manufacturer forged ahead, introducing frequency-specific directionality in the mid-2000s, and then beam-forming technology in 2010 with the Ambra product.  This latest feature, which they called "StereoZoom", tightened what the microphones would pick up from 120 degrees down to only 40 degrees in from of the user, creating a superdirectional feature which worked exceptionally in very loud noise.  At the same time, they introduced "ZoomControl", which allowed the user to select directions other than the front.  This feature worked well for people to hear in the car, so you could "Focus Right" if you were the driver to hear the passenger better, or "Focus Left" if you were the passenger.  If you were a cab driver or soccer mom, you could even set it to focus backwards to hear the people in the back seat.  They have since improved upon this with the Bolero products, where they've even automated these new features like StereoZoom and ZoomControl.

Other manufacturers have mimicked Phonak as well; I believe Siemens and ReSound have features similar to StereoZoom and ZoomControl, but Phonak got there first, so I would continue to look to them (and their sister company, Unitron) to come up with new innovations in the future, and continue to be the leader in the application of directional microphone technology.

All manufacturers have hearing aids with fair working directionality.  I said "fair" to mean that we do not have a perfect system yet when it come to directionality.  The directionality indexes differ by only a few dBs from manufacturer  to manufacturers, and this few dBs are probably not noticeable by a person with hearing loss!  The newest technology is one that the two hearing aids work in tandem, via bluetooth, to improve the directionality index through their algorithms.  If you have trouble in noise, maybe you should look into this newer feature.  I know that Phonak and Unitron have this feature, but if you have a prefered brand, you can look it up online.

You've asked a difficult question that has no easy answer.  All manufacturers of hearing aids are offing directional mics and they all seem to do a very good job with voice clarity in noisy environments.  I can't really rate one manufacturer as better or worse than another. But as a provider I can tell you that some manufacturers offer more control of the directional function than others.  The goal in programming hearing aids is to achieve the optimum settings for individual patients.  Setting directional capability is a part of the process.  The tighter the forward directional spectrum, the less the background and/or lateral sounds are amplified ... therorectically this offers better one on one communication.  This setting sometimes needs to be tested and reset depending on the environment and the patient.  Siemens is one manufacturer that allows the patient to adjust the directional spectrum via buttons on the aid or via an app on your smartphone.  Overall, most aids do a very nice job self-adjusting to most environments.  Having the ability to overide the automated adjustments may sometimes be beneficial.  Just be sure that your provider fully explains the directional capabilities and makes adjustments for your comfort in the varying environments that you frequent.