A 21" LG CRT TV. It's on the crappy side for CRT TVs, but it gets the job done. I once put my Duo on my in-laws big screen LCD (can't even recall the brand, but one of the big makers), and I nearly stabbed my eyes out. I won't be doing that again.
With all due respect to other humans, my personal opinion is that anyone who thinks an SD game console looks good on an LCD probably has a different definition of "good" than I do. A lot of people who watch SD TV stretched to 16:9, or listen to music with pack-in Apple headphones, or drive a Dodge Caliber, or eat White Castle, or wear dollar store sweat pants...they'll say these things are "fine" or "good enough", but I think its really the inability to tell the difference that helps them stick up for this junk.
What sucks about the LCD thing is that I can't think of any marketing incentive that would ever drive the OEMs to fix this problem. With all movie content coming from set-top internet devices and all game consoles going HD (when the Wii dies, that'll be it) they start removing composite/s-video/component from all new TVs period. Then what we will play TG-16 on? Maybe the same TVs we are now since new electronics are unreliable junk...but some day all the 90s CRTs will be dead, and we'll have to be modding our Duo Rs for HDMI.
I guess, realistically, we only have to bother with keeping this stuff going in our lifetimes. When we're gone in 60 years nobody will give a shit about 16-bit gaming. In the meanwhile we need to learn how to fix old TVs as well as the systems that we run on them.
The alternative of course would be super high performance upscan converters built (competently) with gaming in mind. Something like the X-RGB series, only better.
I've recently to fiddle around with Ootake, and it does a decent job of scanline emulation. I'm thinking that once my Duo goes for good (it's starting to get there now with the dry caps issue), I might just build a cheap HTPC and hook it up to a modern TV via HDMI and have do my retro gaming that way in the near future.
And yeah, White Castle is absolutely disgusting. On a random note, I'm missing the burger joints in Cali -- Islands, Tommy's, In 'n Out... Hell, even the mid-tier stuff like Carl's Jr & Wendy's is helluva lot better than vast majority of the stuff they have down here...