I dunno if it's Top 5, but has anyone ever seen this shootout from the TV show "Millennium", Episode 3.04, "Closure":
I remember seeing this episode was I was about 13 and thinking it was the best shootout I'd ever seen. Of course, I also hadn't seen "Heat" at the time, nor had I heard of the North Hollywood shootout, so I didn't recognize the influence. But I watched this episode again more recently when I did the page for "Millennium" and found that this shootout still holds up well for me, in spite of all the other (bigger-budget) action movies I've watched since. When you first see the two gunmen step off the bus with their riot gear on, I think it's pretty badass. Also, the sound design is excellent, so when they open fire with their XM177s, you definitely can feel the pounding of the automatic fire on your subwoofer.
(Although on a side note, it's kind of ridiculous that the BGs are walking around bare-armed and none of the cops try to shoot them in the arms. Especially since the cops in this episode, unlike the LAPD in North Hollywood, have rifles.)
Anyway, I would say this is one of my personal Top 5 shootouts, outside of movies that are part of the "canon" for gun enthusiasts.