Hey dude.
1. Which are the most played duel maps?
Depends on the game mode but Turbine is pretty popular. Frozen, Ot, Academy, also come to mind. This game has a massive map pool and people tend to gravitate towards different maps, more so than in the old days where we only played like 5-6 different maps.
There tend to be maps that people play /less/ instead of maps that are super popular, if that makes sense — but there are a handful that are definitely played a lot. Try all of them!
2. Is there a player ranking or competitive ladder?
No official ELO system exists in Sauerbraten but there are community-created ones that pull data from different servers.
3. Are there texture packs? What about better (or more) player models?
Yes, and no. The game is open source so you can really do whatever you want, but any custom content is client-side only. Messing with textures/models could possibly fuck up your game and in some cases prevent you from playing.
4. Any other useful tip you can give to a newbie? :s
Get good noob.