I would have to say that the Pyro and Scout updates are probably tied in my book. In my opinion those are the two updates which really succeeded in making unlockables that were alternatives, and not simple upgrades. The backburner is
extremely useful in the right spots, and the normal flamethrower is
extremely useful in others. The flare gun is a perfect example of an unlockable, and the axetingisher tends to find better use as an actual weapon than as a backup weapon in case you run out of ammo, as the normal fire axe is.
The scout is probably even better, with me myself changing the layout constantly to suit what I want to do. If I plan on running past SGs, than the bonk gets put into use; if I want to be more defensive, I usually use the sandman. I personally find the Force of Nature to be better than the scatter gun, but I have realized this is only my opinion; I see scouts with it in their inventory using the scatter gun all the time, and doing quite well. So I'm not going to label the FoN as overpowered.
In my opinion you can really tell when an unlockable is good by how frequently you are changing your layout of weapons. Medic managed to get two upgrades and a really, really good side-grade. The ubersaw and needlegun (I can't recall the name right now) replacements are simply better than the originals, but the Kritskrieg is a perfect addition. If you're dieing too fast to get a normal ubercharge, than switching to the KK often helps you assist your team more. But the Needlegun and ubersaws don't have enough drawbacks. If you were to ask my I would call for a damage reduction; something to make them a little more impractical in self defense.
The only problem I have with the heavy update is that the KGB are simply better than the normal fists. If the KGB were fixed I would rank it up there with the Pyro and Scout updates.
And I don't really care for achievements or maps (I generally stick to 2fort and Turbine; probably because I like CTF a lot), so those doesn't really effect my opinions on any of the updates.
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But the scout update is really terrible....sandman is really messing this game up, not a lot of scouts are present nowadays but come on, to stop ubers is just way too powerful. Also you can just bonk a guy and he stays like that for 10 sec....just too easy if you ask me.
Scout: Valve screwed us over with this one. this update was nothing like the others. The Sandman is a horrible creation that deserves to be called hax. BONK! is a stupid concept made worse by not being able to use your weapons. the Force-a-nature is annoying if you're against it but isn't actually THAT bad.
I've given up on playing the scout at all and I refuse to use the sandman unless it's for an event.
Now I find this interesting. People seem to hate the sandman, but I've rarely been killed by it, and even that small chance of death quickly degenerates to 0% when I'm with teammates as the game expects me to be. The recent nerf it has received has lowered scouts' chances of killing me even further. The most I ever find it is annoying, and honestly I die at the hands of scouts w/out using the sandman more often than scouts w/ the sandman. As a scout I find it impractical; I usually try to run past enemies rather than take them down, as a result I never take them head on. The sandman prohibits me from double jumping, which I quickly find out I use quite often whenever I find myself using the sandman and unable to jump when I need to. Sure, with the sandman I can stop
one guy from taking my out while I run past him, but it doesn't do me much good if there's five guys right next to him.
As for bonk leaving a scout defenseless, I can only relate a memorable time where I was able to stay in the enemies intel in 2fort for around 15 minutes while I was
very low on health, and being attacked by like five different people. I mainly did it using only bonk. Invulnerability coupled with a short cool down time, and then using it again had me running around while they pointlessly shot at me. I would then turn a corner and kill the one guy who came after me with a point-blank FaN and then use bonk yet again. It just kept repeating itself, with my getting health whenever I got the chance. All in all I'd say that bonk is
very useful, but it's
very situational.