Depends on the Gundam. But I suppose you mean the best.
If it is the originals in comparison I give Gundam a SLIGHT edge. If you are comparing Robotech to any of the following:
-Mobile Suit Gundam: War in the Pocket
-Mobile Suit Gundam: Stardust Memories
OR ESPECIALLY
-Mobile Suit Gundam: MS 08th Team
-Gundam Wing
I'm going to have to say Gundam by a landslide. It really was the pinnacle of sci-fi wartime anime in the 1980s and while I think the medium def. grew far beyond and far more sophisticated than Gundam in the 1990s, that is when my two favorite Gundam shows, Wing and MS 08th Team, were produced.
So I vote Gundam. The characters for the most part are richer and there is something more interesting of it being about humans who have become disenfranchised with each other after generations of space colonization in a self-destructive civil war as opposed to fighting invading aliens (which feels too black and white).
But Robotech is much better than G Gundam and Gundam Seed which were both awful. I've only seen the first 4-5 episodes of Gundam Zero, which was pretty good, but not enough to comment.
Did I mention how much I love Gundam Wing, though?