I've locked different toons at many levels to experience what it has to offer. I've found the most active communities are the ones you would expect: 19, 60, 70, 79, 80. 19 is still how it used to be, everyone is in full heirlooms/greens with all the best available enchants. At 60/70/80 you have full top-of-the-line end game pvp sets, replacing heirlooms for resilience, and using the next expansion's best gems, etc. I find the most fun times to lock exp, permanently as a dedicated twink, or just to stick around for some extra fun, are at levels 19 and 70. Level 70 twinking has some incredibly fun pvp, and if you can find some friends, it's super fun to do arena at 70. Hope that helps!