Not every level-up reward is created equal. Some of these are essentially just filler, which makes sense for balancing reasons. On the other hand, many of these rewards can be a huge power spike, making XP extremely important.
Gaining an important upgrade, a vital skill, or a relevant enchant can often make or break a build. Knowing this, you can actually adapt and make certain decisions during your run to better align with the path of rewards. Let's take a closer look.
Level 2 - Bronze Skill

Bronze Skills offer a huge boost to the early game and can often be a deciding factor for winning the first PvP Encounter. For example, adding an extra +20 Damage to a Weapon with Multicast is a massive difference.
However, in a lot of cases, players won't actually reach level two on the first day. The only way to make this happen is to choose XP events twice, leaving just one vendor to buy items from. Whether or not you decide to go for an early level-up will usually depend on your starting item and the merchants offered.
If the first selection doesn't include the choice for XP and you're able to pick up good items from the shop, it's likely worth choosing the XP option the next two times. However, if the XP events are given earlier, it'll be a gamble since you don't know the final shop. Having a good starting item can help make this decision much easier.
Level 4 - Upgrade Bronze Item / Bronze Skill

Both options are a valid choice depending on what you have. If you've got a good Bronze item that you intend on keeping, the upgrade is likely the correct pick. In the later days of the game, Bronze items start to become quite rare, so having a guaranteed upgrade now ensures you avoid this problem for the selected item.
If you don't have any worthwhile items to upgrade and/or if the actual upgrade value is minimal (like a small Damage increase), then another Bronze skill can be a great improvement.
Level 5 - Silver Skill

Silver skills are often where you can start to better shape your build. Many highly synergistic skills begin at Silver tier, including ones that rely on time to scale.
The range of importance is a lot larger than the initial Bronze skill, as there are many more skills in the Silver pool. Sometimes you can get a build defining skill, other times you get junk. Though overall, you've got decent odds at finding at least something useful. It may not benefit you immediately, but it's a good way to pick up a synergy skill for later.
Level 7 - Upgrade Silver Item / Silver Skill

Upgrading from Silver to Gold can be a big deal depending on the item, and in most cases by now you should have at least one priority item that's a mainstay of your build. If not, another Silver skill is a reasonable choice.
It's also worth noting that Bronze items can be upgraded as well, and that can be worth it over a Silver item depending on how your build is shaping up. After all, as mentioned before it becomes challenging to find Bronze upgrades later on. So this can be a way to bring it up to speed.
Level 8 - Orlin (Gold Skill) / Gold Tier Skill Vendor / Gold Tier Skill Vendor

Gold skills are a huge turning point for many builds. While this level-up also offers two random category-specific skill vendors, do note that many of those skills can appear at Bronze or Silver.
Orlin is your sure-fire way to acquire a Gold tier skill, but of course with a larger pool of options. Despite the wide variety, there are a ton of good ones, meaning after a reroll you're likely to find at least something you like.
Level 10 - Random Enchantment (Known) / The Artist (Unknown Random Enchantment)

Hands down the most important level-up of all is at Level 10. Gaining access to your first enchantment can turn a good build into an insane build. It can also be a way to salvage a poor run, as certain item/enchant combinations can be game breaking.
Being able to have a strong start and continuously defeat the highest XP PvE Encounter gives you a chance to outpace your opponents' levels. This could mean the difference between you having an enchant in a PvP fight when your opponent doesn't yet have one. Keep a very close eye on XP when you are approaching Level 10.
Level 11 - Gold Tier Skill Vendor / Gold Tier Skill Vendor / Hero-Specific Skill Vendor

More skills are a welcome addition, and what makes this one special is having access to your Hero's exclusive skill vendor. Many of these are incredibly powerful, build-defining skills.
What you decide will ultimately depend on the type of skills you're after, as some of the Hero exclusive skills only fit into a specific archetype. Check out our Items | Skills page for more details.
Level 13 - Upgrade Any Item / Orlin (Gold Skill)

This level is where things really start to come together. While you have the freedom to upgrade any item, you'll often want to get your best Gold item to Diamond. Certain items can even be a night and day difference between those tiers, so it's important to ensure they're at their best.
From Level 10 onward you should have an item in mind worth upgrading. If you don't, you'll want to be on the constant search to find one. While a Gold skill is a solid alternative, this is the first reliable upgrade to Diamond. Plenty more skills will follow, so unless your board is already in fantastic shape, it'll be hard to pass this one up.
Level 14 - Adira (Diamond Skill) / Gold Tier Skill Vendor / Gold Tier Skill Vendor

While many rewards have filler levels between them, Levels 13-14 are both huge power spikes. This level offers a guaranteed Diamond tier skill, many of which are absurdly strong and even meta defining.
Unless you're in search of a very specific category of skill, it's impossible to recommend anything else. Diamond skills are insane, and if you don't see one you initially like, ALWAYS reroll for another!
Level 15+ (See Full List Above)

As for everything else, you can refer to the full list above. However, the one thing I want to note is the one difference between the "Gold Tier Skill Vendor" rewards.
Unlike Level 8, Level 11, and Level 14 that are partially predetermined (Orlin, Hero-Specific, Adira), the later rewards are more random. It could simply be three different "Gold Tier Skill Vendors", or alternatively one vendor is replaced by the "Hero-Specific Skill Vendor".
Regardless, plenty of great rewards live at Level 15 and beyond. So if you happen to keep progressing through a long run, there's plenty of stuff still in store!