There is no universally "best" or "coolest" outfit in the game, as it is a matter of personal taste and preference. There are a number of ways of going at assembling an outfit. To show off wealth, one could buy the most expensive combination of items, whether they match or not. That being the case, one of the party hats plus any of the treasure trail clothes would fit. An example of that would be the Blue Party Hat plus the Saradomin Vestments (minus the miter). Many people like the color Red, for some reason, and wear the Red Party hat plus full Dragon armor plus an appropriate cape. You don't need a party hat to look good, though. Members like cavalier hats, holiday items and wearable rewards from quests. Free-players have to work on a shorter list, but still enjoy holiday items and some treasure trail items.
Most of "eye-catching" outfits have a theme. The "coolest outfit" may be the personal arrangement of a person's accomplishments with a deeper meaning than just the right colors. Skillers wear a skill-cape and match it with clothes or armor in a similar theme.
If money isn't your thing, one could earn an outfit from any of the mini-games, including the new Mobilizing Armies.