A couple top brands not yet mentioned: Timberwolf, American CLS. They have some more expensive models than those you'll find in big box stores, but they can be customized for whatever you might need (log lift, multi-way wedges, output tables, conveyors, etc.).
As mentioned, Super Split sets the standard for kinetic/flywheel models.
For a big box store hydraulic model these days, aim for a cycle time around 11 seconds or faster. Once you get above 2o or so tons, speed is more important than force IMO. I'm also a fan of the in-beam log cradle found on DHT, Oregon, and SpeeCo (CountyLine, Black Diamond, Huskee, et al.) splitters.