Skiller - If you enjoy slaving skills. U can join a Clan to make it more fun. But will be completely skill based, nothing combat wise (Pking, Bossing, Etc.)
Ironman - If u want to do it by your own, do it. I wouldn't recommend unless you're insane.
1 Defence - Good for low level pking for sure. Fun and fast paced. The higher the combat, the more difficult.
20 defence - I would get if you're only into clanning. It's 20-25 for the maxed defence in pure clans.
42-45 Defence - A lot of questing. Once you get the skills and questing out the way it's a fun build. I loved it, I just got 99 attack/strength and had to kill dhers all day. My whip max hits 40 while their axe hits a 70.
70+ Defence - If you enjoy skilling, bossing and pking its a good build. The pking is very slow though, and bossing will be easier the higher def you are. You basically unlocked every piece of armour in the game.