So, we all have played those games where low levels suck because you can't do much or, conversely, where it is like using iddqd and idkfa at the higher levels (god mode and infinate ammo for the non-doom folks). So I ask the brain trust: what, in your opinion, is the ideal rank/renown zone to have a fun and challenging deathwatch campaign?

I've never made it beyond rank 1/initiate before so I am very curious what the rest of the game has to offer.

I have minimal experience with running high-level campaigns, so I can't tell yet where does it stop being fun, but on the low end, above rank 2 or so a player already has access to enough nifty talents and wargear to be able to pull off some awesome things.

Challenge level is a whole different story, it has too many variables to give a 'one-size-fits-all' advice. The thing is, even Rank 1 characters (themselves already veterans in the narrative) are insanely powerful in the 40k setting, and if the players are new to 40k roleplaying, I'd definitely run a few adventures at rank 1 or 2 to familiarise everyone with the existing skillset of the characters. If you're worried about low-level combat balance, check the free adventure PDFs (Final Sanction, Oblivion's Edge) for a few ideas and enemy scaling: they are well-written, and while Oblivion's Edge is a bit pushing it (a handful of marines taking down a Hive Ship), Final Sanction proved to be absolutely absorbing for my new group at that time.

I don't know whether this answers your question, but specify further, and I'll gladly tell you what I know