I would recommend the following, I've been using these for quite some time now:
1. Arturia CS-80V - it's the only plug-in that sounds analog enough to be used on it's own in a track.
2. Arturia Moog Modular - For synth basses, this things a monster, I wasn't as impressed w/ their Minimoog V.
3. Spectrasonic Atmosphere is still a fantastic synth, it's like have the ultimate synth sound module in Logic.
4. The PowerCore 01 that comes with TC PowerCore is pretty slick, but you'd have to buy a PowerCore system. I've heard good things about the Access Virus as well.
5. MOTU MX4 - by far my fav!
If you're doing hip-hop, you may just want to get some really good Rhodes and Drum Samples for the EXS24.
Hope that helps.
UltraQuick, easy & cheap option for drum machine / sampleplayers / synths all in one - reason 3 (although the rewire thing with logic is a pain to set up in the first instance)
plus - easy access to huge amount of refills, a lot of the free ones on the propellerheads site are decent
I used it with PC/cubase for years (just moved to mac/logic myself) & the best things i found are
a) its so easy to learn & use you can work at lightspeed b) it really doesn't tax the cpu so you just dont have to worry about track counts etc etc. I've often found after an hour or two that I've created a rack of 40+ instruments in the creative process & then I just delete the ones with ideas that didnt work out.
1 tip - if you do go down this route get the updates for both programs (ie 7.2.1 & reason 3.05)- the rewire implementation (at least in versions 7.2 & 3.0) wasnt right
GForce Imposcar (Or the entire Gforce synthline for that matter...cheap and EXCELLENT Sound.)
Spectrasonics Stylus RMX....is an absolute must have for hiphop production IMO. (As well as Trilogy and Atmosphere)
Another thing you should look into is a plugin call Vintage Warmer by PSP Audioware....Download the demo and try it out, you'll love it....I know it's not an instrument, but if you want phatness...there ya go.
the sounds that come with reason are just straight garbage. There's really no point to getting reason when you already have a sampler/sequencer/synths in logic pro.
If you want the truth, get a high quality rompler like sampletank xl 2.1 w/sonik synth, plugsound pro, hypersonic ( never used but heard good things) etc.
BTW I made a video tutorial on how to use reason with logic pro the correct way and give perf. tips, even though I NEVER use reason anymore.
In answer to the "instruments that work well with L7" part of your question, any instrument that is in Audio Unit format will be usable with Logic 7.
You've got a lot of recommendations so far, but those are mostly based on personal favorites and the type of music you've said you're trying to make. Don't feel as though you're limited to what people are recommending...if it's an AudioUnit, it should work.
That being said, here are my recommendations...hehehe.
GForce Minimonsta - Excellent emulation of the classic Minimoog. I prefer it to Arturia's Minimoog V, but opinions vary. The coolest thing it has that Arturia doesn't is the ability to do realtime morphing from one patch to another (the Melohman feature). Anyway, the Minimoog/Moog Voyager is a very hot sound for synth leads in hip-hop, so a Moog emulation will be a good thing to have in the arsenal.
Spectrasonics Trilogy - Every kind of bass sound you could ever want, from raw and squelchy synths to jazzy acoustic uprights.
I'm not going to make any drum recommendations, because the sound of your beats is very much an identity thing for hip-hop, and I have no idea what your sound is. There are lots of options out there...Stylus is great, but anything from Microtonic to BFD to GURU to Battery might also be your thing. Check 'em all out before you make a decision.
There are actually a bunch of good sounds in Reason, mixed in with a bunch of bad sounds. Pretty much the same is true for the synths and sampler in Logic. Plenty of bad presets, at least for the kind of music I like.
I've found lots of great stuff in Reason, and as software goes, it's the least troublesome music program I've ever used (and I've got quite a few.)