This or frontiers

[Beavis The Monkeyspanker]

Title

[Juken]

If you want a more open world atmospheric adventure - Frontiers
If you want more linear levels with better boost gameplay and mini-games - This

[Mez Koo]

Two very different experiences.

If you want a new experimental open world format then go with Frontiers.

If you want a more traditional stage format with boost style then go with Shadow Generations.

Although if you haven't played Sonic Generations before then definitely get this one first since it comes with 2 great boost games unless you're not a fan of boost.

[Skorge]

this

[Dolphyn]

While I like Frontiers a lot more, I'd say play Generations first if you've never experienced the original. Collectively with Shadow's campaign it's the best Sonic game.

[SneakyBeat]

This if you never played vanilla Generations.