Units that give team buffs are pretty nice to have around. One of my personal favorites
is was Graham's Union Flag Custom, since its a cheap unit (B rank) and has an insane boost pool when Assault command is active. (rock+booster extension+Assault Command= win) Just fly around kiting people and let your team mates hit them while their backs are turned.
A similar unit is the Union Realdo (Taribia) its also a rock with booster extension, but its W3 is a 3 shot burst that inflicts slow on opponents. Really useful for slowing down foes so that your allies can take advantage of their temporary lack of mobility. Boost zero is also nice. Dahgi Iris and most Doms have them. The main thing to watch out for when using these units is to be careful of people with Vaccine on them.
Tallgeese III has an awesome team buff as well as long range flashlight beams. Full Armor ZZ also has a nice buff, with a focus more on survival. Heck, most S ranks that give team buffs are pretty good.
Tallgeese II provides both Assault and Defense command. Some units with radar are useful too. (like the CR GM Ground type and Zaku I command)
ReZEL Commander
is was one of my favorites as well, having Assault commander and excellent ranged game for a scissors unit.
....
I miss this game.