Thru this video I started to appreciate the mobile suits from the Universal Century Series (esp. the units from Gundam Unicorn). You get to see here not only the old units from the earlier UC period, but also the improved versions of it.
Which actually brings me to being curious about the evolution of the mobile suits. Below are just some examples of these. Some are improved versions of its predecessors while others are alternate versions of the units from different series.
Images via Toysdaily