The ME N scale stained ties I bought were fine, if you wanted modern 7x9x9ft ties. If you're using ME flextrak the ME ties were the same size, which makes them my choice for mainline track. I used their HOn3 ties for bridge ties in a few places, where I wanted something taller than the standard 10x10. Stand them in edge and they're about 14-15 inches tall in N scale.
Clover house has several sizes, but the prestained ones I've seen are all "aged" ties, not freshly creosoted. Fine for older track, not so good for a well maintained mainline. Of course, they can always be painted or stained to suit.
I haven't tried the Kappler ties, but the few pieces of lumber I have from them look good. If I ever need more ties that's probably where I'll go. They're only available unstained, so will need to be colored. Also, their bridge ties are 8x8, so I would probably use stripwood and cut my own, as I prefer 10x10 for most bridges.
