Play by Post: What are the best systems for it? What are good house rules for it? And what are the best examples of it?

I guess I put everything in the title...

Well, I'm wondering how one might improve on "PbP" style of playing RPGs (ideally something with at least some crunch to it). My experiences with it hasn't been too great to be honest, mainly when it has come to battles. So to reiterate: What systems best lend themselves to the concept? What house rules (with no limit on how much they may change) could be added to make it a much smoother process? Finally, I would personally love it if someone could link me to what could be considered the most successful "PbP" game they've seen. What I mean to say is one with both stability and longevity. You can also ignore my questions and just describe personal experiences if you'd like.

Thanks!

My personal experience has been that theater-style live action rules (Mind's Eye Theater comes to mind as an example) tend to transfer well to PbP play - such rules are often designed to work without the need for dice, or the need for a GM to arbitrate every single conflict or task resolution.

Any game that regularly calls for iteration between GM and player to figure out if there's a success is going to be a problem in PbP. An exchange like this:
Player: I want to do X!
GM: Do you wish to do X using Skill A or Attribute B?
Player: Skill A. My B is cruddy.
GM: Does anyone want to assist? Every person who wants to Aid, please make a Skill A check.
Player 2: Does my Power Q apply?
GM: No. Just straight Skill A.
Player 2: But I got Power Q for just this situation....

May take a couple of minutes at a gaming table, but can take *days* in PbP.

The most successful combat I have seen in PbP has not been played out round-by-round. Instead, the GMs ask for general descriptions of how the characters approach the fight, "I am (recklessly aggressive/normal/defensive), I have a goal of (killing many bad guys/rescuing the princess/protecting the party mage), I really want to use (my Power Q if that's applicable)..." And the GMs weigh the submissions and write up a scene that describes the action. This fight resolution, however, is very GM heavy and GM subjective, and so requires GMs that are highly dedicated, have free time, and are trusted by the players to a large degree.

Hmm, yeah. That's the direction I've been leaning towards, except I think rolling would still play a big part. I think it is important to let players roll for their actions even they are being mostly interpreted by the DM.