Textra, because I can set the size of a pix to 600 kb or 300 kb. I have an AT&T microcell at home and with the native LG G3 app, it didn't seem to reduce the pix size sufficiently to allow the MMS to go out.
Textra is uncomplicated, easy to use.
I was using Google Messenger because it showed full messages on my G Watch, but I went back to the default LG app when I installed Coffee SMS for Android Wear. If you have an Android Wear watch that app is a must have.
I like chomp sms a lot, but I stopped using it when group and picture messaging stopped working right. I never worked out if it was a chomp issue or T-Mobile issue.
I like Textra a lot, but it doesn't work with Agent's Drive Mode and that's a deal breaker. Currently playing with AwSMS which was mentioned earlier in the thread (thanks for the tip). Pretty good looking app, although some bug fixes are desperately needed and an update is due this week.