The thing about soloing is that it depends a lot of what you are doing. For example, many Mesmers prefer to use Traveler's runes (or other movement speed or swiftness runes) in the open world because they don't high swiftness up time. This is especially true if you are doing stuff like daily gatherer/vista a daily farming of rich ore nodes, etc. In these cases, you want to sacrifice DPS in order to gain speed.

If you are just fighting trash mobs, you might value spiky damage more than long term DPS, because many classes can spike a trash mob to death in less than 2 seconds. By the time you reach the next trash mob, your cooldowns are back and you can spike it to death again. Some of the highest sustained DPS builds may involve condition damage, but those actually take longer to kill a few trash mobs. When I play open world with my Mesmer, I use Traveler's runes and a shatter build because I am usually exploring, mining nodes, and killing a few trash mobs on the way. It is terrible for world bosses, but I do that on another character.

If you are solo, but you are likely forced to work with other players in the map (ex: the Silverwaste bosses), then something like a Phalanx Warrior might be best because you'll expect un-optimal allies (ex: a bunch of people in tank gear, etc) and a lack of might stacking.

Generally, they are not too much different from the meta dungeon builds. You typically have to go into the trait lines that give DPS traits (power, precision, ferocity, maybe condition damage/duration), especially if there is a trait that gives something like power and precision. All the meta builds do that too. It usually ends up being pretty obvious what the trait choices should be after that. Then you may want to make some tweaks to get stuff like movement speed, even at the cost of some DPS, because that is so handy in open world PvE.