What does the Mechanic skill do?

Thanks, I didn't realize you had to activate cheatcodes first.

Shoot, I'm not that good of a player, but I'd say this:

TOP 5:

1. Heaven Can Wait (We Hope!!!) (Must be the Christianity that comes out in me.)
2. Poles Apart (Not really backroutable, at least from my perspective.)
3. The Crankshaft (As someone said earlier, the lack of bashers makes the level more interesting)
4. It's Hero Time! (One of everything, pretty tough and pretty fun!)
5. Feel the Heat (Short and simple, not a build-a-thon)

BOTTOM 5:

5. Walk the Web Rope (Long and tedious)
4. The Ascending Pillar Scenario (Long and tedious again...)
3. Hunt the Nessy (Long and tedious AGAIN!)
2. All or Nothing (If you know the trick, or have one of those newer versions, this level's a dud.)
1. Tightrope City (Just BLAND and boring.)

Actually there is a backroute for Poles Apart, which lets you complete it 100% in under 1 minute.

Yeah, my Just A Minute solution is a few seconds faster than his. The replay should still be up on the Lemmings File Portal. Basically you just have to make a lemming dig to the right of the first basher, so that the basher sinks a few pixels when he gets to the digger, which lets the digger dig down far enough to prevent the lemmings from escaping to the left but still lets him turn back into a walker when he bashes to the right. You can do this a few times to trap the lemmings further and further into the wall.

Crazy 1 can actually be completed 100% with diggers and blockers.

I verified for you that Taxing 17 can be solved 100% with builders and bashers. I uploaded the replay to the Lemmings File Portal along with Tricky 20 and Taxing 21, 26 and 29.

Continuing on with the Taxing levels, 21 can be completed (100%, obviously) with climbers, builders and miners. I was trying to get a builder-basher-miner solution using the wall jumping glitch but I didn't have enough builders to make it work.