That's rather difficult to answer because people mean different things when they say a game is "deep".
Some mean that there is a lot of content, in which case many of the best candidates at this point in time are games that were not developed exclusively for VR but retrofitted for it. Two of my favourites in that category are The Solus Project and Serious Sam: The First Encounter. Both have large campaigns, Solus is mostly exploration-based and Serious Sam features very fast action and active traversal of large maps. Arizona Sunshine is a VR-only game with a campaign of ~5 hours, and it's fun, but I don't think it's particularly deep.
In another case you might mean deep as in the complexity the mechanics allow for. In that case, you need to look at something like Anyland, which basically allows you to build anything and attach scripts to it to make it do mostly anything. Of course, there's a discussion to be had about whether something without a win condition or explicit goal is still a game.
Finally -- and this is what I mostly mean when I say "depth" -- you can consider to what extent and in how many ways a game allows you to improve your skills in it in order to get better at playing it (this can range from things like reflexes to learning strategies for specific situations). In this category I personally think that
Raw Data stands a notch above most other games at this point.
I really just play one character (and each of them has very different gameplay you need to learn individually), but even so there is a whole lot of ways in which I feel I have improved since starting the game (and can improve further!):
- How to perform basic attacks. As in, for example, how to physically throw my sword to hit distant enemies, or precisely throw it at the right angle and speed to inflict a large amount of damage to a group of enemies in front of me.
- Understanding the attack patterns of each enemy, and knowing which of them are most dangerous and should be taken care of first in different situations.
- How and when to use special skills (which are on timeouts) most effectively.
- Mixing in fast traversal without "overheating" and with minimal interruption to whatever else I'm doing.
- Learning the individual map layouts and which enemies spawn where and when.
- Strategic decisions like which towers to build (and repair) where and at what point in time in each level.
And what I find particularly nice about Raw Data is that it doesn't just offer these options to get more deeply into it -- it actually has a difficulty curve that, on normal, requires you to at least make some progress in each of these areas to proceed through the later levels.
