Kind of reminds me of the "What is RIGHT with Modern Sonic?" thread, but hey, I'll bite.
Adventure-
My first video game, and my first Sonic game. I feel I will be absolutely
hooked on this game for life. I replay through the story fully at least 4 separate times a year, and that's if I don't try often. Honestly, the Level Design is my favorite in any 3-D Sonic, it has my favorite Soundtrack of the 3-D Games, favorite 3-D portrayals of characters (Tails anyone?), and overall just a fun game to play.
Adventure 2-
Back in the day, I loved the crap out of this game. Almost as much as Adventure 1. Overtime I came to realize how much I hated some of the stages, and realized I wasn't having any fun with the Tails/Eggman Knuckles/Rouge gameplay. Knuckles Emerald hunts were rather bearable in SA1, but here it just didn't work. Don't get me started on the Space Emerald Hunts. I completely fail at those. (And really, Knuckles jumped like, what, 2 yards to break the Master Emerald? How did they get to SPACE?

) It's still alright though, and I still find myself shedding a manly tear from time to time at the ending.
Heroes-
I kinda feel neutral about this game. I hate Tails' voice, Shadow's return (solely for making me feel stupid for shedding the tear at SA2's ending, other-wise I could care less. AKA, not too big of a deal, but worth noting), and some of the levels. Not because the design of the levels was particularly bad, I just didn't like a lot of them.
However, I did like quite a few songs from the soundtrack, and Team Sonic/Shadow's gameplay enough for it to be somewhat worthwhile to revisit from time to time.
Shadow-
Ugh. I didn't play this until late last year, when I bought it used for like 15 USD. Dark grungy graphics (and one of the few bright stages is that Internet Circuit thing that gives me head-aches), portrayals of Sonic characters doing things I never would have imagined them doing in my younger years (I mean, slight language, guns, wha), and a rather unmemorable soundtrack. Some of the stages just seemed so confusing. I had to have wasted half an hour on one of the ARK stages with Shadow and Maria just wondering where the hell I was supposed to go.
'06-
Oddly enough, I enjoyed this game. Enough to complete it 100%. I can still admit it was a bad game, yet...
To be completely honest? I rarely EVER experienced any glitches, of any kind. Honestly, I was looking. I found a few noticable ones, but unless people here just really have a pet peeve with it, or I was lucky enough to avoid such inconveniences, it was a problem I rarely encountered.
As a few others have stated around here, it felt like the spiritual successor to the Adventure series, from the level design, to the story. The sooundtrack was rather good as well. This is by no means a game I would recommend to a friend, but I find myself playing this game every now and then. (Namely; Wave Ocean, Crisis City, and Kingdom Valley as Sonic.)
Unleashed (360/PS3)-
This game was great in my eyes. It was a true delight to play, with some of the best visuals I had even seen on an Xbox 360. The soundtrack is the 2nd best in the Modern series (In my opinon), and the Daytime gameplay, while having the whole "HOLD X TO WIN + QUICK TIME EVENTS = PROFIT" formula, I still enjoyed it. I played it a lot over the last year.
And then there's the Werehog. I never liked the idea of it. I never want him again in a game like this unless there's more Platforming and much less Combat, but I
still enjoyed it. For how out of place and un-needed it truly was, I found it to be rather enjoyable.
By far it is my 2nd favorite 3-D Sonic.
Unleashed (Wii/PS2)-
This game was alright. Too many Werehog levels for my liking (really, there were like, 4 or 5 consecutive levels for Eggmanland.), but just alright.
Advance 1-
I've had this game for a few years. I never could beat one of the last levels (the Base level, can't remember the name), but it was a pretty fun game.
Advance 2-
Haven't played it.
Advance 3-
I found this at a yard sale a few months back for 3 USD. I can't say I like it. I don't know what it is about it...I just feel like I never truly know where I am in a level, and that frustrates me to no end in Platforming games.
Rush-
Rush Adventure -
Haven't played either of those. Hope to sometime though.
So yeah.