Canon have produced multitudes of xx-300 EF lenses, many of them garbage, a few are notable.
Cheap grunge
All of the 90-300 and 75-300 non-IS lenses fall in this category. Also in here is the 100-300 F5.6.
75-300 IS (1995)
This one is notable, but only for being the first IS SLR lens (ironically, nikon had played with VR in a compact camera before this, but felt it wasn't a success). Optically this one is nothing to write home about, closely related to its non-IS cousin above.
100-300 USM
Optically this is barely any better than the cheapy stuff, but it's got ring-USM and is a bit more nicely built.
100-300 F5.6L
Unlike its non-L cousin, this one was optically decent. Horrid mechanics (AFD focusing, push-pull zoom), but it ended its life being sold at non-L prices. These days though you'd choose a 70-200 F4L with 1.4X TC for better optical performance without the mechanical compromise.
70-300 DO IS
A mixed bag. This was supposed to be the lens that brought the promise of DO to the masses. Had DO been everything that everyone hoped then the new L lens would never have been made. Optically reasonably sharp, but hampered by poor contrast and weird bokeh.
70-300 IS
A mid-range lens to replace the previous IS zoom, but offering far improved optics, definitely a step above the cheapies, if not quite with lenses like the 70-200 F4L. Very well liked lens, if only canon would give it ring-usm.
70-300L IS
The new, white one. As yet, no one really knows how this one is. Likely to replace the DO, as they are too close in price for both to live.