I would try to get to 40D, minimum. Sure, it's got some features you don't need, but it's also got some really nice ones that the earlier models don't have:
14 bit vs 12 bit images. This literally gives thousands more color gradations.
Self-cleaning sensor. One of the pains in the arse with older models was keeping the sensors dust free. The self-cleaning sensors aren't perfect, but work surprisingly well (especially if you avoid ridiculously small apertures that just cause loss to diffraction anyway).
Interchangeable focus screens. If you do landscape or especially any sort of architectural photography, it's great to have a "grid" type focus screen.
Higher usable ISO. I used 10D, then 30D, then 50D and now 7D crop cameras. 10D I'd limit to ISO 800. 30D I'd use 1600 if I really needed to, but would have to do extra work on the images in post-processing. 40D and 50D are about the same in high ISO performance and I'd use either of them at 1600 without much concern or need for extra work in post. Now with 7D I'll use 3200 (and with 5DII, 6400). You might feel differently about just what's usable, but the same general principles would apply.... whatever you find acceptible on 20D/30D, you'd likely be able to go about a stop higher on 40D/50D (and, conversely, would need to reduce by a stop with 10D).
Improved AF. The 20D, 30D and 40D all appear to have the same AF system at first glance (so do 60D and some of the Rebel series cameras). However, prior to the 40D the 9-point AF system had only one of the more sensitive "dual axis" type AF points, at the center. The rest are less sensitive dingle axis type (some horizontal, some vertical). Beginning with 40D, all 9 points are dual axis type. So the peripheral AF points are more usable in the later cameras. (Beginning with T4i we also see this on the Rebel series cameras, by the way.)
Live View also was a new feature with the 40D. It is a nice feature to have for some things.
Larger LCD monitor on the 40D sort of goes hand-in-hand with the Live View feature. The 40D has a 3 inch diagonal/230,000 pixel LCD monitor screen, up from 2.5 inch/230,000 pixels on 30D, and 1.8 inch/113,000 pixels on 20D and 10D.
There are more differences, above just touches on some of the high points and most used features.
Depending upon what you want to shoot, I might go against some of what I've written above and suggest a 5D classic. The full frame camera is usable all the way up to it's limit of ISO 3200, so it's great in low/available light shooting. It's 12 bit, but "full frame" format and a weak anti-alias filter tend to make up for that. My main complaint about 5D classic (and the reason I never bought and used one regularly), is that it's a bit of a dust magnet and doesn't have a self-cleaning sensor. I was already cleaning my crop cameras more often than I liked, and dreaded the thought of even more of the same with the 5DC. Another negative with the 5DC is a slower frame rate (also not good for sports) and the AF system is sort of a throwback to earlier models, 9-points with only the center one the "better" dual axis type. (Actually the 5DC has an AF expansion mode with 6 more hidden AF points, but that's only usable in AI Servo, which is for moving subjects and something the camera just isn't all that great at shooting. By the way, the 5D Mark II inherited the 5DC's AF system almost exactly.)
Now, if you are mainly interested in wildlife or sports, I'd recommend going with one of the crop cameras. If you are more interested in scenic/architecture shots or low light work, it's hard to beat a full frame camera. The full frame is also very nice for portraiture, and the 5D models have long been favorites of wedding photographers.
On the other hand, with crop cameras (later than 10D) you have a wider selection of lenses. There have been many "crop only" lenses made just for those cameras, Canon's EF-S and a variety of third party, that won't work on a full frame camera. All full frame design lenses will also work fine on the crop cameras. So while there are still a lot to choose among, there is a bit less of a selection for a 5DC, than there is for crop model such as 20/30/40D.
Really any of these cameras are very capable. So too are the Rebel series, if you find a great deal on one of those. The main differences are that the above cameras are mostly magnesium bodied and all have true pentaprisms, while the Rebel series use more plastic parts and a penta-mirror arrangement to reduce size, weight and cost a little. Rebel series also tend to have a lower rated shutter, less weather sealing and some other general downgrades, but still are quite capable cameras and some have neat, useful features you won't find on the more "upscale" models... for example, certain Rebel models have an articulated LCD screen, which can be really nice when shooting unusually high or low angle shots, such as macro photography.
If you can, buy the camera with a lens. That's often the best deal. The 20/30/40/50D often came in kit with an EF 28-135 IS USM, which isn't really a "kit" lens per se. It's actually a darned capable lens with very good image quality, fast accurate USM focus drive, very close focusing ability (near macro), and helpful Image Stabilization. It's also an older film/full frame design, so would be usable on either crop or FF camera. So many 28-135s have been sold, this lens can often be found for $250 or less lightly used (sells for about $450 new). It isn't an L-series in build and sealing and isn't at all wide on a crop camera, but is far more capable than many people give it credit for and makes for a nice walk-around lens. Pair it up with a Sigma 10-20mm, Tokina 12-24mm or Canon 10-22mm and you'll have a pretty darned versatile two-lens kit. If you have your sights on wildlife, eventually add a Canon 100-400mm or a Sigma 120-400 or 150-500, and you'll be "loaded for bear" with just three lenses. Add a set of macro extension tubes to the 28-135mm and you'll be able to photograph those bees, beetles and butterflies, too.
Alternatively, the "kit" lenses sold most often with Rebel series are capable of making nice images, too. They are "crop only", but the EF-S 18-55 and 55-250 are quite common and affordable. With these you'll have moderately wide to a fairly strong telephoto covered. Be sure to get the later "IS" versions at least, as they are optically better than the earlier versions. These aren't USM lenses, so aren't as fast and accurate focusing.... the latest versions with STM focus are better than the earlier models, but you aren't going to find as favorable prices on those since they are fairly recent. You see these kit lenses selling second hand fairly cheap a lot, as people tend to upgrade from them to something better as their skills grow and they feel the need. Just watch which version you get... best is the latest IS/STM... good is the IS... and I'd probably try to avoid any earlier, non-IS.
But the 40D has got me very interested and probably more advanced then the 20D. Has to be way better then my Sony setup I have now.