It's interesting seeing different playing styles reflected in inventory choices! :D
Personally, I don't bother with envirosuits at all, unless I'm going for the Lava Pits, in which case I take just one. Most of the time, I can get around levels without stepping in fluids, and even if I do, I only step in one or two squares, and if I do it while running, I take minimal damage. As for the Lava Pits, there are three envirosuits on that level, so I only need one to get me as far as one of the islands that has one, and then I can use those ones to complete the level. (Of course, I usually play Nightmare, so the suits last longer; if you're playing a lower difficulty level, you might want to bring more) :) Of course, it's worth noting that envirosuits give 25% fire and acid protection to your whole body, so they can also be used to give you an edge in combat. (perhaps against the Cyberdemon?) Personally, I'd rather just avoid getting hit in the first place, but depending on your playing style, using the suits like that might be a good strategy for you.
I usually take one Phase Device, preferably a Homing Phase Device, just in case. I find Phase Devices too unreliable; they take me into the middle of danger as often as they get me out of it. Of course, again, this may be a reflection of the fact that I play Nightmare difficulty, so there are a lot more enemies, even in areas I've already explored. If I have an HPD, I'll often clear the area around the exit first, so that if I run into any trouble elsewhere in the level, I know that using the HPD will take me to safety.
If I'm playing Nightmare, at least two stacks of rockets, maybe more. (They're the easiest way to get rid of bodies, so I want a lot of them) Otherwise, one stack is sufficient, in case I want to do some wall-busting or if Arch-Viles are giving me trouble. :)
5 or 6 med-packs, and 3 or 4 spare suits of armour. (If I have regenerating armour, like the Tactical Armour, then just one spare suit of armour)
The rest depends on what build I'm using, but I like to have a lot of shells, because I'm fond of radar shooting with the shotgun, I like the ability to damage multiple targets with the spread, and the knock-back is frequently useful as well. I'll take maybe 4 stacks or more of shells. (and yes, I use them to the end of the game) I only carry bullets if I'm using either a pistol build, rapid-fire build, or if I'm playing Angel of 100 and I need some back-up ammo in case I run out of other stuff. (I'd rather melt my enemies with plasma weaponry) :) Oftentimes, I'll have a Chaingun in my inventory, but I won't carry any bullets besides the ones already in the Chaingun. I just keep it around in case I need some precision shooting, but I don't have time to reload the Plasma Rifle.
EDIT: And, of course, I'll have one melee weapon, usually the Chainsaw, or in the late-game, Longinus Spear or Azrael's Scythe. If it's Angel of 100, it'll probably be a Chainsword. Maybe someday, if I'm lucky, it'll be the Dragonslayer. ;)
EDIT2: And on an unrelated note, I just played a little more of IWBTG, and I came to the irrefutable conclusion that the designers are assholes. :P