If I had to pick one tuner with the best reputation on the internet it would be EuroCharged. Everyone has nice things to say about being tuned by Jerry. They are super easy to work with, and apparently over 60,000 cars have been worked on by them, check out their blog for some of their interesting projects.
Send in your ECU to clone and tune, use handheld's like MyGenius. TCU tunes, custom tunes, pick up the phone or drop a forum/fb dm, and I'm sure they will work with you. They have great deals at times and offer their services at multiple locations.
Let's back up this G-wagen a bit. I usually jump into the internet feedback first, but I haven't really introduced Eurocharged here. Eurocharged is well respected for tuning ECUs/TCUs for fine automotive brands including Mercedes.
EuroCharged blog is filled with amazing ECU & TCU tuning projects from Lamborghinis to McLarens, and it looks like following their Instagram will get these pushed to you: https://www.instagram.com/eurochargedperformance/ .
In their own words:
Eurocharged Performance, offers many different services in its exclusive and well maintained facilitates all around the world. Everything from the basic bolt on to a monster street / strip car, we have what you are looking for!
EuroCharged warrants their work (I'll double check about TCU tunes -- sounds like no? still not 100%). EuroCharged is often praised for their after sales service across social media and forums. On the very small TunedMercedes Facebook Group several have commented how they enjoy their EC tuned beasts, such as a EC tuned S65.
Jake P, owner of eurocharged has some of the fastest cars in the 1/4 on dragy. He has a gorgeous offshore speed boat that apparently scares women and small children.