The stock firmware is the best, if you can accept the limited feature set and the limitations of the web admin GUI. It's reliable and the wireless has good coverage. I'd try it first and see what you think.
The lack of monitoring tools in the stock firmware is the biggest missing part for me. No traffic graphs, doesn't have a decent system log, and you don't see static clients in the client lists (as I recall). Other users mention other missing features, but I use only the basic routing and wireless and IPv6 with my routers. You also cannot add software packages to the firmware, as some people do, nor can you telnet to the router. There is a secret status page you can look at, but that's basically a dump of various router internals to a text web page, not very accessible to the user. It does have the only record of router uptime, though, so it has some limited use.
When I took OpenWRT (both Chaos Calmer final and Chao Calmer final rc3 versions) for a test drive the other day, it didn't work well enough with my Apple mobile devices. They either could not connect, or the speed was limited. If you don't have Apple mobile devices it could work well for you, although it locks up for some people after a period of days.
I also tried the latest Kong DD-WRT for the WRT1900AC, and that didn't work well with my particular wireless-ac clients, so I stopped using it. If you have wireless-ac clients that work properly with the DD-WRT firmware for the WRT1900AC, that also can work well. Or if you don't have wireless-ac, my wireless-n clients worked fine with it. Again, though, it has the same problems for the long term as OpenWRT, since it's based on the same buggy wireless driver that OpenWRT is.
Linksys continues to market this router as "OpenWRT ready", etc., while Marvell has yet to deliver a wireless driver that's as good as the one that they have in the stock firmware to open source. This could change, given all the versions of this product line that are out there now that Linksys wants to sell, but I'm not holding my breath for something that hasn't happened yet *smile*.