The worst line is normally the line for the fairies. First thing when Toontown opens or late at night are the best times if you decide you really want to meet them. The Princess line can also get pretty long but not as bad as the Fairies. Another line that can get really long is the Character Spot in Epcot. Finally, the greeting trails in AK can get pretty bad; however, how bad varies throughout the day... normally they are worst right after a Festival of the Lion King show gets out. If you get there in the middle of a show normally it isn't so bad.
We just saw both the fairies and the princesses last week on Monday. Tried to get there at rope drop but just didn't quite make it. The line for the fairies said 45 minutes at 10:30. We got in line & were just outside the area where the sign and the tv are. We did have to wait, but it wasn't 45 minutes. Maybe 30 tops. They only take a handful of guests at a time so that certainly contributes to the slow moving line. We saw Tink, Fawn & Terrence & all 3 were great with DD3. It was a neat experience for her. After the fairies, we turned around & got in line for the princesses and waited maybe 15-20 minutes. That line flew.
While at AK, we only did the line for Pooh, Tigger, & Eeoyre. It was right before the parade and there was no line at all. Walked right up to the characters and was out in time to see the parade. I believe the location was right outside Dinoland USA.
Epcot"s Character Spot was probably the longest wait we had for characters. Not really sure how long we waited. It wasn't an hour but I bet we did spend an hour there total, including standing in line and then actually meeting the characters. But, that being said, it was air conditioned in there sonwe didn't mind!

Plus, at the end, we waited just off to the side by the doors while DH got the stroller and got a surprise visit with Belle. She was coming out of the back of the character greeting area in her blue dress and there was only one other family there. When DD saw her she was so excited she RAN down the hall right into Belle's arms. So priceless.
As for meeting characters elsewhere in the parks, you have to time it just right. While the main map and show times guide for a particular park will tell you when and where the characters are meeting, you have to be really careful to get in line before they cut it off. Tiana & Navine, Jasemine & Alladin, (MK) & Lilo & Stich (AK) were all out for a good chunk of the day, but they were only out for an hour at a time, followed by a good 20-30 minute break before starting up again. If you happen to arrive near the end of that hour, you may not be allowed to get in line. The CMs will rope off the line after a certain amount of time and you may be SOL.
And in Epcot, it's even trickier. We saw the Beast in France and Snow White in Germany only because I read the DIS. Not even kidding! Had I not known they made appearance in those countries, we would have missed them completely. They were kind of in off the main path areas and often times the only way you knew they were coming was the presence of a photopass photographer and maybe a handful of people kind of forming a line. There's not really a specific mention of what character will be appearing, a sign just says "Character Greetings" with the times listed as "itermittently" from 11-5 or whatever.
You just never know who or what you're gonna find! lol