Operation Ivy takes a lot of heat for spawning Rancid (a pale imitation of The Clash) and a huge crapfest of awful bands who played pop-punk with horns. However, the "ska" label was somewhat overstated for this band, especially since only one or two songs on Energy actually feature horns (Bad Town is the only one I can recall). And these guys were miles ahead of their imitators in every conceivable way.
What Op Ivy really did back in 1988-89 was combine punk, ska, and reggae (with a little hardcore edge thrown in), and record the whole mess nice and lo-fi--original and totally excellent stuff. Blink-182 and Sum41 fans will cringe at the DIY-level sound quality and Jesse Michaels' untrained voice, but I wouldn't trade Energy for any of their Pro Tools-smothered junk. THIS is punk. Overproduction need not apply.
With 27 tracks, each averaging about 2 minutes, Energy has the punk energy down pat. Tim Armstrong ("Lint") lays down great speedy hyper-ska guitar lines all over the place, and Matt McCall is probably one of the few punk bassists who isn't totally incompetent (am I hearing some slap bass?). The album is chock full of melodies, catchy choruses, surprisingly sophisticated lyrics (especially considering that these guys were merely teenagers at the time), and plenty of groove. At least half of these songs are worthy of becoming standards--the opener Knowledge (Green Day covered it early on, but the original is superior), Sound System, Take Warning (my favorite), The Crowd, Bad Town, Freeze Up..I could go on and on.
Forget the "ska-punk" label--ultimately this is excellent non-hyphenated punk rock, and it should be in your collection.