Some essentials, IMO, are:
1). Sweeney Todd. The Original Cast recording with Landsbury is wonderful, but I prefer the concert performance with LuPone and co! The recording is much more complete, the sound spectacular, the cast great, and the New York Philharmonic is really something. Murder has never been more clever or fun.
2). Les Miserables. I would go with the CRS (Complete Symphonic Recording) over any other recording, as it is the complete show and it is wonderful.
3). Miss Saigon. I think you would like this show, based on what else you have. Again, I would go with the CSR over the OLC. It's complete and features better performances, IMO.
4). Sondheim's Into the Woods is another show I treasure. Famous fairytales intertwined in brilliant fashion. Get the OBC with B. Peters. It's dynamite.
5). If you love Phantom, I think you'd also enjoy Jekyll & Hyde. Go with the Complete Work, on 2CDs, with Warlow and Eder. It's much better than the OBC.
6). I also really like Chess. It's very interesting. The concept work (on 2CDs) is standard, but I like the now out of print Danish Cast Recording best!
7). EVITA! Great show. I think you'd like it. Avoid the film soundtrack and stick with the American recording, with Patti LuPone.

. Ragtime. I love this show. It's an epic tale set to awesome music.
9). Aspects of Love. This is ALW's masterpiece IMO. It is definitely a work that requires effort and patience (I hated it for years). But, if you stick it out, you realize what a wonder it is. It's my favorite ALW show and one of my favorite musicals. It's only available on the OLC, but what a great recording it is.
10). The Witches of Eastwick. I love this show. It is spectacular, and the performances on the OLC are great.
11). Whistle down the Wind. Despite the bad southern accents, the music is both beautiful and rockin.' The OLC is quite good.
12). The Woman in White. Expensive now because you have to buy it as in import, but it is really something.
That's all I can think of right now. Of course, I love all these shows. If you really like the shows you have now, I think you will like all these. Ragtime and Aspects will be the biggest challanges, possibly, but they are both in my top 5.
You can find most of these, if not all, for sale used on Amazon.com. I prefer to look for used recordings, because you can get more for your money.
Hope this helps!
"What the hell happened to you? You look like a Make-A-Wish Kid. You know, I just knew you were gonna bring shame on this new family of ours, and it just figures you had to go make yourself over into some heroin-shootin skate board chic on the only day E! could interview you!" - Cherry Cherry, on her daughter Mary Cherry