Most stuff? Realistically? Just high quality 10x's on chest, and you're good.
Then, depending on what you know about the terrain and how thick or open it is, you decide if you need to back-down or upgrade from there. And if you want to deal with bringing the heavier stuff.
And when considering that thought.. determine if you're willing to travel into some far off direction where you've spotted something.. where it will likely require pitching a tent for the night once you arrive near to where you were looking at. And If that's not your speed? And you'd only ever be bothering to advance upon a target if you perceive the distance to be traversible within a single days time on the trip too and single nights time to get back out, then I'd say try 15's first. You'll likely be happy with them. Really it depends on the terrain/habitat you're gonna find yourself in.
If a lotta steep high elevation gain/drop up-and-down between where you are, and where that game animal is.. I believe it's likely you may find that 15's are all you need if your game plan is trying to do at most an overnighter at worst while en route.
Places I'll go usually DON'T have easily accessible water in there, so that tends to limit lofty multi-day goals like that. Unless you have the ability to pre-pack-in some water into a mid-way point on prior scouting missions or something. But... some folks live places where the mountains have all kindsa water access, and more power to them! If that's your scenario, you at least have the luxury of considering those heavier load-out options more readily.
For me if the place I'm going is such that I think long-distance glassing is a for sure, fore-gone conclusion, I'll drop back down to a set of compact 8's I have for hand-checks on the chest or jacket pocket, and the 15's for on the tripod. Or if very open country, I might even just do 15's only. Depends on how much water/fluids I have to carry. If it's 5L, or up as high as 8L sometimes in brutally hot months, I'm finding ways to drop other things I might not need. And like I said before sometimes if you've done homework and you know from cams they are in there.. but it's thicker and you can't really glass around due to habitat, on prior scouting missions you bring along unopened gallons to stash out there. In order to buy yourself more time to remain like on an ambush sit. Especially when the whole thing is in a sea of shoulder-high Greasewoods/Chaparral in-between spots dotted with Oaks where draws creates seasonal water flow lines.
Also besides weight there is space in the pack to consider, when you don't have water readily available. Since you tend to need to bring more water in with you, so more pack space getting taken up. This possibly could influence your available choices on what you could realistically bring in with you. And THAT also gets more limited as age and injuries start limiting you even more.
