All of the times the main characters crossed over that I could think of from the pre-streaming days. I deliberately left out minor characters like Sarek and Kor for time/to make chasing down all the leads easier, maybe another time.
0:00 - tng 1x01, Leonard McCoy
0:35 - tng 5x07/5x08, Spock
1:40 - tng 6x04, Montgomery Scott
3:24 - ds9 1x01, Jean-Luc Picard
4:08 - tng 6x16, Julian Bashir
5:03 - tng 7x21, Quark
5:48 - ds9 3x09, Thomas Riker (I mean he is kinda Will)
7:07 - voy 1x01, Morn and Quark
8:29 - ds9 3x19, Tuvok (Mirror)
8:53 - voy 2x03, Reginald Barclay (see also 6x10, 6x24, 7x06, 7x20, 7x25, and 7x26)
9:39 - voy 2x18, William Riker
10:06 - voy 3x02, Hikaru Sulu and Janice Rand
11:36 - ds9 5x06, Crew of the USS Enterprise
12:44 - mov 8, EMH
13:22 - ds9 5x16, EMH
14:05 - voy 5x06, Geordi La Forge
15:11 - voy 6x24, Deanna Troi (see also 6x10 and 7x06)
15:50 - mov 10, Kathryn Janeway
16:53 - ent 4x22, William Riker and Deanna Troi (Also features archive voices of Data, Jean-Luc Picard, and James T. Kirk)
Reg and Deanna got cut down to a single voyager appearance each since they became somewhat recurring characters for that series (especially Reg) and I didn't want to end off on an overdose of Reg/Deanna.