My list would have to go with
Somewhere in the Between- Streetlight Manifesto
Keasby Nights - Catch 22/Streetlight Manifesto
Let's Face It - The Mighty Mighty Bosstones
40 oz. to Freedom - Sublime
GNV FLA - Less Than Jake
Big Daddy Multitude - Mustard Plug
Everything Sucks - Reel Big Fish
Strictly Rude - Big D and The Kids Table
there you can have fun with that for a while
While I'd argue that 40oz is not "ska" per se, I'd also arguethat anyone who doesn't own it must go get it now, I'll even spot you $5 towards the purchase.
Keasby is a great one (either is great, but go with Catch because it's the original), as is Everything Sucks!
I'd also recommend Upbeats and Beatdowns by Five Iron Frenzy (not the best album, but their most ska/ska-punk influenced album and it's solid. They evolved to become basically a dork rock band with horns)
The Toasters greatest hits collection, entitled Don't Let the Bastards Grind You Down is great and one of the most clear examples of the third wave sound.
LTJ's Losing Streak is my favorite ska-unk album ever and The Pilfers's Chawalaleng is crazy good too.