Ok I have a couple that I've found really useful over time:
os_forceid - make it a perm module? Useful if someone has a BNC and after a long netsplit the nick gets Guested.
uhm, absolutely not! This is actually a huge security problem. Yeah, yeah . I've heard all the sob stories about how annoying it is when you're buddies are afk and lurking for days on end and they get guested every 2 minutes. HOWEVER, let Services do it's job. If it's really so important make sure users are updating their /ns access lists. IF a user is connected from one of these addresses they won't be guested in the first place! Honestly though, which is more important? your buddy staying in channel for several hours while they are away for the weekend? or recovering his/her nick and perhaps some settings because someone accidentally forceid'ed the wrong person?
Being a popular module doesn't make it a good idea. Security should come first at all times.
Secondly, being a 3rd party module makes no difference. It just means you don't have to download it when you download Anope and since you already have it, no need to worry about it. There are far more useful top 10 modules that aren't even in the core.
Prevent Channel founders or QOP's being akicked. I'm not sure if this is perm yet just thought i'd mention it.
Services can't PREVENT people from being kicked, this is strictly an ircd thing.
ns_saregister/sagroup & /cs_saregister - really useful one I've found. I'd limit it to services admins+
The beautiful thing about being modular is to allow admins to customize their services without having to know how to edit the source code. Couple this it the fact that you can download hundreds of 3rd party modules that work just as well as 3rd party modules as they would core modules, it makes no functional difference as to which they are.
The core should be kept as tiny as possible and in my humble opinion some of the current core modules could be 3rd party modules to make it even smaller.
Asking for 3rd party modules to become core modules may seem a trivial thing to you, but it's not to the development team that now must support the modules instead of their respective author and the only thing it saves is a few minutes time for you to download a few files but could add several man hours maintaining modules that only a small hand full of users actually use compared to those that don't.
Maybe auto-assign Chanserv or Botserv to a channel as it's registered? Or any bot (editable by config).
These is a 3rd party module for that already, for 1.8 atleast. More than one to be honest.