Welcome to yet another one of these. So let's see, I talked about Icon of Sin maps being awesome and everything as well as several maps that didn't quite make a good impression, so now let's all take a look at the starting levels that did make great impressions and all. Note that it isn't limited to the E1M1 or MAP01 slots, it can pertain to ExM1 slots as well!
Great: Entryway (Doom II)
People, you knew it would show up somehow. Entryway is probably the most played map in any IWAD, perhaps it could be beaten by Hangar depending on how many people played (Ultimate) Doom more, but Entryway still has one of the most recognizable starts. The two zombies on the steps facing away, the green halls, the room with imps in cages, the room with zombies on the blocks, this is all in memory for most players. Even the secrets are not forgotten, that chainsaw behind the start, the courtyard with the shotgun, and the rocket launcher secret. This is also still the most popular deathmatch map. What's not to love?
Not good: Hell Keep (E3M1 of Doom)
There honestly is no beating around the bush. Hell Keep flat out sucks. Your challenge of using only a pistol to fight off imps, then a cacodemon or two, then finding a shotgun over the falling bridge are one thing, then there's the really dumb corridors that are incredibly thin with demons in them. But their AI is easily exploitable in those confines. Then a few more rooms, one with imps, and bam, you exit. That's it? Anyone can make a map better than this.
Great: System Control (TNT: Evilution)
It's important for a first map to leave a good impression, even if it's not the biggest challenge in a megawad. Should a gimmick be introduced first, or a rather enticing prize? That's what System Control does. It's my personal favorite MAP01 in an IWAD because you get a berserk right at the start and can feel great about punching things right from the get-go. And it's not all that challenging, even with all those dumb backtracking moments and stuff.
Not good: Hell Beneath (E4M1 of Doom)
Though a challenge is welcome, one must wonder how far a challenge can go right from a starting level without being unfair. Hell Beneath, at least on Ultra-Violence, has skinned numerous players alive. Especially completionists, who know that there's really not enough ammo to handle everything, especially the four barons of hell that are in the NIN secret. Drained players also had to deal with some annoying teleporter ambushes at times.
Great: Shuttlecraft (Icarus: Alien Vanguard)
The author of this map just so happens to be the author of a controversial map of Evilution, Habitat. Yet he made this and it's actually one of the better maps out there, particularly for this megawad about ships, planets, and simulations. Doses of realism at the start lead to Star Trek-like areas, with alarms blaring, wading through lava, and then that awesome highlight of a mutating chapel. I like what I see, and the level isn't too long or anything terrible either. Good things definitely came out of this author here.
Not good: Pistol Panic (Community Chest)
The first Community Chest really did not age that well. Pistol Panic is a map that tries a gimmick with being only able to use the first few weapon slots. That's...okay I guess? How about you explain why there's all these random voodoo dolls in the level? It makes it slightly challenging to avoid shooting them, but their presence is kinda unwarranted and doesn't make this map feel any better. It's already slow enough with pistol-only combat.
Great: IGNITION! (Hell Revealed II)
For the harder sets, the first map should start with a bang, and boy does IGNITION! do that. Action start, non-linear setup, hard enemies like arch-viles, cacodemons, and mancubi already out here, and even a couple of actually difficult secrets show players what to expect for the rest to come. And the author, Jonas Feragen, implements his style quite well both here and elsewhere in the megawad.
Not good: Into the Gate (Hell Revealed)
The original Hell Revealed is quite bonkers in style. Into the Gate is like IGNITION! in that it shows what's to come horde-wise, although they are quite staggered in approaches. That teleporter near the end is basically the main idea, but wow, you can already anticipate the demons, the zombiemen aren't actually that tough. And the last teleporter closet with imps only opens up as you approach the exit. It's incredibly easy to miss. That cacodemon outside on the window is just silly, and so is this whole level.
Great: Werewolf Moon (Hell Ground)
Combining atmospheric locales with good gameplay is a challenge many mappers face, and Eternal makes this look like cake with the gothic locales of Werewolf Moon. It's ambient at the start, find your way in, then the player's gotta deal with the insides of the area, which are of course under control of the hellspawn. Yet the hellspawn took precious care of the building, not laying any hellish fingerprints over the lavishness of the halls and such. It's wonderful to look at and to play.
Not good: Dropoff (Realm of Chaos)
Then again, when maps make a first impression, sometimes they don't do good in one aspect. Dropoff sucks in the design department, only having some good gameplay but having a lot of gray just about everywhere. There's gray walls, terminals, and a few crates for two rooms and a hallway. It's even worse when you consider the ending sequence can be broken preventing players from ever reaching the exit.