Sonic's forays into the 3D space have commonly produced mixed results, but they have a lot of fans nonetheless. What's your favorite of the bunch?

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Bonus question: What are your hopes and expectations for the upcoming Sonic Frontiers?

Sonic adventure 2 followed by Generations then Colors. Back in 2001 that opening City escape level was incredible from a graphical and gameplay perspective.

Sonic Colors for me.
My hopes are high for Frontiers but expectations are low. I'm not expecting it to be great.

I've only played 3 of the 3D Sonic games completely, of those I'd rank them Sonic Adventure 2 > Sonic Generations > Sonic Forces. That's not to say that Sonic Forces is bad, I actually liked it alot more than most people did, thought it was a solid 7.5 or 8/10 game, but the other 2 are better. I've also played a demo of Heroes and started playing Sonic 06 before a gamebreaking bug halted my progress, so I don't really feel comfortable ranking either without properly playing them.

Adventure, but that's mostly nostalgia. The best would probably be Sonic Generations. Also liked Secret Rings a lot which I feel is underrated.

Mine is still Sonic Adventure 2, with Sonic Generations a close second. I don't expect Sonic Frontiers to be very good, but I am curious.

In a few hours Sonic Forces will be my first.

So I guess that.

I only played two so far, being Sonic Colors and then Sonic Lost World, both earlier this year for the first time. I found Colors to be super tedious und annoying while Lost World felt fantastic and amazed me with the level design, so that's an easy win for me. I'll be playing Sonic Boom next, somewhere down the line.

My biggest hope is that Sonic Frontiers will be an actual good game and stop making people criticise every tiny little detail. I can't stand all this bitching and crying from the so called fans anymore.

Colors

Adventure 2. I've never come close to enjoying the others I've tried (Adventure, Heroes, Lost World, Colours, Forces) as much as SA2. I've probably sunk well over 100 hours into that game in my lifetime.