Is there an Achievement tool that tells me which achievements I should go for next in Team Fortress 2?
e.g. the easiest one or the one that that will give me my next milestone.
A similar service for Item Crafting advising is tf2crafting.info.
An achievement adviser and a crafting adviser are two different beats.
Most TF2 achievements are unrelated to one another... which is some feat, considering the game has nearly 400 of them.
The Official TF2 Wiki has achievement guides broken up by class (and general). These guides break down the achievements by difficulty. It's recommended that you do the easier achievements first in order to get the associated weapon unlocks.
I am currently building a site that will show you the next easiest achievement you can go for to take you to your next nearest/easiest Milestone achievement.
I will update this post as I get further along.
Update: I've decided to create an achievement advisor at: www.tf2next.info It's workable, still need to put more work into it, but I've used it myself sometimes to look up what achievement to go for next while playing during lunch break when we are so lucky.