This is also a question of mine, so I look forward to reading your responses.
The best festival experience I had was The Sycamore Film Festival, which appears to no longer exist. They flew me back to central Illinois from Cali and treated all the filmmakers fantastically. I'm keeping an eye on it, in case they decide to bring it back.
I know Sean will recommend the Flyway Film Festival, which I haven't had the honor of being accepted to yet, though not for lack of trying.
I've screened three times at Illinois International, which was admittedly a pretty small event put on with a consumer projector in an independent theater without heat, but they were always very nice, cheap, and had full Q&As with everyone.
I'm waiting to hear from the MiddleCoast Film Festival, which sounds like a great time here in the Midwest. I've attended the Chicago Critics Festival, which is held at Chicago's coolest theater (The Music Box). Donnacha's theater will be hosting the Naperville Independent Film Festival, which is located in the Chicago burbs.
In LA, Sean and I saw a premier at Cinema at the Edge Independent Film Festival, which was small-to-medium but was presented very professionally to a packed house. If you're looking to go big-time, AFI is a massive event for all of LA every year, it seems. Definitely an upper tier festival in presentation and pricing, but it's "legit".
I think I'll be going to Tromadance this year, which definitely caters to a pretty specific niche, but promises to be loads of fun. And it is free to submit, so no harm there.
Most of the others I've been in were small ventures that have since faded away, but will update if I remember more, or if anything develops RE: a Sunset Rock festival screening.