Level suitability
Here are my opinions about the suitability of the various levels, for kill 'em all speed runs.
Level 1: The Pillar of Autumn
Fabulous. Short and intense, it makes an ideal starting point for getting into these runs, and there's lots to think about in regard to tactics. The enemy are generally ahead of you, so you don't often need to look around for isolated enemies to polish off, which makes things nice and simple. If only there was a shotgun available! *sigh* Check out BCM22, KSR3 and KSR4 for examples on Legendary.
Level 2: Halo
Pretty good, but your time is liable to depend quite strongly on how good you are at blitzing enemy troops as they dismount from dropships; and in particular, how good you are at killing the Elites (e.g. by tagging) while they're still aboard or still recovering from a dismount. Get it wrong and you can be in a world of trouble, out in the open with a couple of angry eight-footers eying you up as target practice for their plasma rifles.
Level 3: The Truth and Reconciliation
Excellent, with the enemy encountered thick and fast. On Heroic in particular, it's ideal as a non-stop blasting rampage. To get in the correct frame of mind, shout "Charge!" at the beginning, then proceed to spit death in all directions as you blitz your way through. I've had a great time with this. The plasma combo (plasma pistol and plasma rifle) is especially good, if you want things visceral.
Level 4: The Silent Cartographer
Excellent, though if you're playing it entirely on foot as I like to do, there are a few long spells of running with no enemies. I've yet to properly explore doing runs with hog use (I plan to), but check out KSR2 for a foot-based example on Legendary. Oh, and watch out for all those nasty Jackals when making your way up from the bottom of the main facility. On Legendary there are 22!
Level 5: Assault on the Control Room
Excellent, and potentially a big challenge due to its length and the fact that dying means game over, something which ratchets up the tension as you get deeper into the level. Particularly good for trying different weapon restrictions; there are so many possibilities. You can also enjoy trying to tackle it on foot, something which may or may not be a new experience for you. Check out KSR1 for an example on Legendary.
Level 6: 343 Guilty Spark
Relatively poor. Enemies are woefully sparse to begin with, and then you're in a confusingly maze-like facility. You'd better be familiar with the route! Flood tend to be scattered about in the rooms, and often at different levels from yourself, so you have to look around a bit to seek them out, which can make things feel a bit slow. Also, the final section is just a dash to the tower, unless you've made the challenge tough enough to force you to be a lot more wary about running straight through all those gurgling Flood.
Level 7: The Library
Excellent, though if you like to keep on the move you might not fancy it much as you're obliged to hang around in many places to deal with wave after wave of spawning (possibly a lot more spawning than you're used to, especially if you've previously been in the habit of just running like hell). But I like all that and find it quite absorbing. Favoured combo: pistol and shotgun.
Level 8: Two Betrayals
Excellent, but probably one of the toughest levels to tackle at speed. All those Flood with rocket launchers make it very dangerous. Watch out for the mob that rushes the second generator room!
Level 9: Keyes
Pretty good, especially if you're going to be using the shotgun. But the spores can be darned annoying in the outdoor section, making it hard for you to recharge your shield. Also, the special-ops Elites at the end can be murder. You'll want to really think about your tactics, to take them out fast. I had to add quite a few level-specific rule details for this level to try and optimize the playing experience, so there's a fair bit you'll need to keep in mind.
Level 10: The Maw
Pretty good, though once you get to Engineering or the Armory, the kill 'em all aspect is dropped and it just becomes a bid for escape (though you'll still have to kill plenty of Flood just for self-preservation). The escape potentially allows you to benefit from good driving skill, but surviving the huge Warthog jump near the end seems to be a lottery, and it's frustrating as hell to be killed like that through no fault of your own, when you've just done a sterling job of battling through the level at high speed.