I prefer circular bases, and have standardised on Renedra 25mm for Oathmark and Frostgrave commonality.
Combined with Renedra Sabot bases they are easily moved.
The comment earlier about circular bases allowing you to rotate the figures can make it a lot easier to get them not stabbing the soldier in front. It is going to be really useful when I start on the Mantic Gargoyles with those wings everywhere.
With my Quarantine 37 order I also ordered FireForge Byzantines for Oathmark. They had a new to me 20 x 20 square bases with rounded edges.
There are another interesting base in the Fireforge Northmen Cavalry.
They might work for cavalry in skirmish games better than the 40mm circular bases.
I would prefer 25x50 mm capsule bases, but currently I have only seen them made by Warlord.