Note that I tend to play face-smashers of the ECL-having, high durability variety. So this might color that.
Level 1 (Sometimes): Inn basement. Can help the weaker classes/races get past a stiff wind blowing you over.
1-4: Undersigil. Standard, easy. Sometimes take 4-5 in the Mindflayer part if I'm no ECL.
5-10-ish: Brux. Can be painful if you can't see invis. Avoid treants unless you've DR, regen, or a party. Be prepared to die some if you're soloing on a non-super race. It'll hurt.
10-15-ish: Ice or Fire plane. I find the Fire plane to be underrated. If you have decent AC and any fire resist, it's about 1k+ XP per 10 minute tour at ECL 10-13 or so. Otherwise, ice is fairly standard. Try to have some ice resist. Cone of Cold is a bitch.
Some people prefer Astral in this range. I find Astral to be 'get gangraped by ten Gith that'll roll 20s eventually'. If you have a party with some durability or CC then Astral tends to win out. Soloing, I'd not do it.
15-20: Minauros. You really want regen, DR, or sneak attack immunity here. HiPSing, sneak attacking Dogai hurt like a BITCH. If you have that handled this place isn't bad. If I'm running a build that gets shit on by HiPS (Dex-AC based folk), I might go to abyss right away.
20+: Abyss. The endgame for 80% at least of the players here. You'll learn to love to hate this place. Can be soloed, bosses need parties.
25+ (If lucky): Maze/Carceri. These places rarely get ran. If you find a group and are competent, go for it. Not all builds should tread here.
wrote:15-20: Minauros. You really want regen, DR, or sneak attack immunity here. HiPSing, sneak attacking Dogai hurt like a BITCH. If you have that handled this place isn't bad. If I'm running a build that gets shit on by HiPS (Dex-AC based folk), I might go to abyss right away.
Word of help / Warning, Minauros is one of those areas where the difference between cutter and gatecrasher is huge. The Dogai mentioned above are relatively easy, but they gain HIPS when you put things on Gatecrasher. This makes them a lot more dangerous. If you can't handle HIPS well, consider staying on cutter the whole time you are in the area.
Going back in time to early levels, you shouldn't be grinding at first level. If you run all the static quests (muriel/khazeet/wildlife intruder/mercuria berries/all around the hive), you can get L2 easy and most of the way to L3, plus earn enough coin for a cheap magic item or two from a PC merchant, before you ever step foot in undersigil.
I also like doing some early quests and then spending 2-10 in Gatecrasher Brux. It's one of my favourite ways to get things going, where possible.
Here's a link to another thread with leveling insights: Hello!
Having a friend along helps a lot, esp. if they are playing something cheesy. Our leveling record is 1-28 in 8.5h, although Ms. Chassiel had existing bonus exp.