When you have full control over the log format you can use with Sumo Logic, we recommend human readable key-value pairs whenever possible. JSON and XML are also acceptable, though XML is harder to read and parse than JSON.
Whatever you use, make the fields easy to parse, including the time-stamp. Also, make sure to add all the "tags" you want to track, such as, module, priority, src_ip, etc.
For example, from our own Prod logs, we use the following:
2012-08-16 13:39:36,979 [metrics] INFO com.sumologic.util.Reporter - com.sumologic.util.scala.Aggregator.Buffer[/usr/sumo/stream-19.5-3/logs/stream.log]-Size-Meter (bytes/s) count=78847, fifteenMinuteRate=0.26, fiveMinuteRate=0.11, oneMinuteRate=0.00, meanRate=1.06
This is a key-value pair log, which is easy to parse and easy to read in the Sumo Logic Web Application.