Right, here's the experts advice (eg, mine)
GH1: A great set list of songs however can be difficult to play as the HO's and PO's (Hammer On's and Pull Off's) can be tricky as harmonix hadn't figured them out yet. It's only on the PS2, so you'd need one of them to play. Nothing that special about the guitar.
GH2: A good set list and improved HO's and PO's. The guitar's pretty awesome in it's Red and Black SG form for the PS2. This one's also available for the Xbox 360 for which I'd recommend it for. It has improved graphics, more songs, more characters and achievements. The Xplorer controller is pretty cool too. This game on the 360 also has quite a wide range of Downloadable Content on Xbox Live including lots of the songs from GH1.
GH Rock The Eighties: Another PS2 option here. It's exactly the same as GH2 however the songs are all eighties 'hits' and the characters have 80's outfits. No special guitar.
GH3: Probably the best gameplay wise for just guitar. The set list is huge and although the duels can be very annoying, are very satisfying to finish. The les Paul controller is probably the best one they've ever made due to the fact you can store them better as the neck comes off. The HO's and PO's are at their best here, which is very important as it's easier to play them and more authentic to those who actually play the guitar. PS2 version is good, but is better on the X360 or PS3 especially when the PS2 guitar isn't the Les Paul. The Wii version is of similar quality to the PS2 game graphics wise, and the fact that you have to insert the WiiMote into the guitar doesn't help push the price down. A lot of
DCL available for this, including full albums such as Death Magnetic by Metallica which is also compatible with GH World Tour.
GH World Tour: Probably the best GH to date due to the flexibility of the game. The drums are quite realistic for a controller, although the new guitar has it's problems, such as a squeaky strum bar and notes being picked up on the new touch pad even when you haven't touched the thing. Although the touch pad is actually quite fun to use, especially on Hot For Teacher. The set list has some gems in it but a lot more songs which don't make much sense being in the game. This game is the best on the Xbox 360 thanks to XBL for online play. I've heard bad bad things about the PS2 version (my mate had it the day it came out, he hated it then waited for the Xbox 360 one to come out the week later and bought that). Haven't seen it on the Wii but can only presume its like the PS2. A fair amount of DLC available for the game and probably much more to come. This game also has an awesome character and instrument creator (taken from the Tony Hawk's games) which is fun to play with.
Rock Band: Great set list and the original Drums/Bass/Guitar/Vocals game. Guitar is awesome with the second set of higher neck buttons (so you can play like Slash in the solos

). The drums are good, however the tapping of the drum sticks can be quite loud, and aren't very intuative from an actual drum kit. The pedals can be broken easily however Harmonix have said that the US kits have been improved, and I think they could supply replacement parts. The PS2 and Wii versions aren't able to have DLC so they have Track Packs which you can buy in disc form. At it's best on the 360 but now also available on PS3.
RB2: Just like RB but with a different set list. The instruments for this game are meant to be improved, but I'm not sure, as they aren't actually available easily in the UK. All the DLC for the RB games are cross compatible, so any you bought for RB are available instantainiously in RB2 and all the tracks from the first game can be burned to the harddrive for a small fee on the Xbox 360.
GH On Tour: Great game for the DS, also can be very painful to play for some people if you don't have the right size hand. Graphics are good for the DS and the songs available are pretty fun. There's only 4 buttons on this game which might be easier for some people.
GH OT Decades: Just an extension of the 1st on the DS. New set list and venues, and you can share songs on 2 player between the 2 On Tour games.
Hope that helped. And BTW, I didn't look any of that up.