Here is a selection of several tests from different tester. All operators have a good or very good rating. Te differances are in details like price and services. The overall winner is FlixBus:

OperatorOverall Rating*test.defb-bwtgFB-Atlasgetestet.de
FlixBusvery good (1,8) Winner1,81,62,061,8
IC-Busvery good (2,0)2,0---
MeinFernbusvery good (2,2)2,22,12,192,3
ADAC-Postbusvery good (2,4)2,02,4-3,0
BerlinLinienBusvery good (2,4)2,03,02,502,4
DeinBusgood (2,6)2,72,42,753,0
Public Expressgood (2,7)2,62,9--
Touring/Eurolinesgood (3,2)3,13,13,44-

Ratings:

test.de from Stiftung Warentest: Rating form July 2014

fb-bwtg: fernbus-bewertungen.de : Rating from 12 July 2014.

FB-Atlas: Fernbus-Atlas from Mobil in Deutschland: Rating from Mai 2013.

getestet.de from aha.de Internet: Rating from May 2014

*) Overall Rating

0,5 to 1,4 = excelent, 1,5 to 2,4 = very good, 2,5 to 3,4 good, 3,5 to 4,4 okey and 4,5 to 5,0 poor 
This is not an own rating from fernbusguide.de website.