Everyone has their own ideas on this. At present I use four different mitre saws, depending on the job, but in terms of accuracy I have to say the Festool Kapex knocks every other mitre saw I have ever used into a cocked hat. The problem is the price.
I personally detest 12in/305mm mitre saws and will only use them when a 10in/260mm saw can't handle the task (which is rarely in my experience) - they weigh too much, they are too big and bulky, they suffer more from blade warping (especially when doing trim cuts where only one side of the blade is in the material - so called "dust cuts"), and on long bevel cuts the cuts can wander because either the heads tend to "squirm" a bit, or the arm flexes. When I do need a large saw I find myself having to check and recheck every important cut. That's based on using DWS780, Milwaukee 12in and Makita LS1219 saws in the last few years. 10in/260mm saws I find a lot more forgiving.
BTW my cordless saw is a Makita DLS110 - bit on the big side for a 10 incher, however I find the fence, as on all the current Makitas, is a bit iffy. I liked the old deWalt DW717 apart from a few issues there seemed to be with the blade guard and the lack of the shadow line technology. Haven't tried the replacement DWS727 yet, though