Tap water will do for most bugs and dirt.

If you are lucky tailgated a Japayuki bus spewing oil from behind then your windshield is drenched with oil and its raining hard at pit dark evening on a 2-way national hiway, brake and parts cleaner and plenty of tissue/ newspaper will be very helpful.
This happened to me resulting to zero visibility and glare from the oil, good thing i always bring brake cleaner and tissue/newspaper, and luckily a spare wiper blade re-fill since it is due for replacement also.