Here is how and what I have hooked up. Not all are optimal but most are pretty good.
NES: composite also hooked up VGA with Dreamcast NES emulator
Colecovison: composite (via mod)
Atari 2600: composite (via modded colecovision with expansion module)
Genny and CD: Svideo (via mod)
32X: composite (haven't modded it)
Atari XEGS: composite
TI 99/4a: composite
Dreamcast: VGA (looks amazing even better than component for some reason)
Playstation: HDMI played through ps3 or played on PSone with built in LCD screen.
Xbox: Component (HDMI for games compatible with 360.)
Gamecube: component via Wii
Wii: HDMI played through Wii U.
Notice I don't have anything hooked up via RF. If I can't get it at least it in composite I don't want it. My apartment has a ton of interference for some reason when I try to hook it up RF. Maybe its because of all the game systems I have hooked up at one time but I'm not sure.
My set up is pretty similar to Wallyworld. Everything that is 6th generation or before is hooked up to a CRT with RGB scart if available, composite if not. I can't imagine using RF now. Newer consoles are on an HD TV.
Megadrive 2 with 32x & CD
Wii U and X360: HDMI
It depends. I haven't seen the Retron 5 in action to know how accurate the emulation is, but many of us would prefer the original hardware because it feels more authentic. Clone consoles suffer from the theoretical conundrum of being faced with the task of imitating entire systems in all their complexity and with all their unique flaws--it's a lot harder than it sounds. Virtual Console titles require a unique emulator for every single game because each game tales advantage of the hardware in different ways. Many NES and SNES games are super tricky to emulate properly because they have coprocessors built right into the cart.
Yes, because a snes/genesis is going to look horrible on a flatscreen regardless.
From what I have seen/heard, the contrast can sometimes be off and the scanlines aren't that authentic looking. I am not a fan of video filters but they may be preferable to some. This article is worth looking at, plus Game Sack did a thorough review of it a few months back. RGB output from original hardware on CRT is still the ultimate. Second would be using original hardware on a HDTV with an XRGB-mini Framemeister or suchlike. Third would be emulation.
ptdebate: Can't be sure of the exact model without checking, but I have a Sony Trinitron widescreen. Approx 27" I think. It has two scart inputs plus one separate composite/s-video input, so with a couple of Bandridge 5 way scart switching boxes plus a composite switch box everything is available without having to untangle cables! I have switchable extension leads as well to cut down on power consumption.