I have a 6" x 24" long Fiskars I bought as part of a set along with a cutting mat & rotary cutter many years ago. It's my favorite. I have a few O'Lippa squares, which are fine, but their marking lines are limiting.
Because they are so easily available for purchase, I have many, many Omnigrid, some green, most yellow. Omnigrid rulers just o.k.; I'd trade them all in a heartbeat for Fiskars like the one I have. I find my Omnigrid 24" square very useful.