My list in order of "best"
1 Combine HQ
2 MC Blitz
3 UCAV
The Combine HQ is by far the one that brought the most depth to the game along with its related tournaments
It involves participation of all members when it comes to defend and improve its level and technologies (a bit like UCAV, BUT combine wide)
It also opens up lot of challenge and extras for both, combines that doesn't own an MC and those that does. It needs to be handled with care as it come with some mechanics every serious combine willing to progress need to learn. It is also a dangerous place to be if your combine doesn't have the required strength to defend it properly. Best to have it at level 5 for quite some time before thinking about moving forward with it. It requires good knowledge of approximate combine attack force in order to go out there and take one down and also how much defense you should stack in there. When you think you know all about HQ... Think again... you might learn some more to it :) Where players evolve, whether it is their total troop force or strategy used against your enemy, so does how a combine should handle their HQ
MC Blitz comes in 2nd place but very close to the HQ, one of the main reason is because HQ has been here for quite some time now, where the MC Blitz just started its first appearance on Soldiers Inc Plarium server
Like the HQ, it involves a LOT but on so much different level then the HQ. Whether is it for a combine with many MCs or one with few and even a combine with no MC, You got to make sure you fully understand its mechanics before making any decision on what direction your combine will go before and during the event. I'm not going to spit all of out just yet but most should know by now that it can create serious vulnerabilities on big combines where on the counterpart, it can open up lots of opportunity for combines without an MC and would like one. Willing to make a move? Suggest a game plan to your CEO and Officers, have it approved and prepare yourself!
UCAV comes in 3rd place for me
A LOT like the HQ, it involve a lot about improvements but more on a individual basis. That doesn't mean you shouldn't care nor improve it... It has a LOT of benefit when used properly depending what type of situation you are facing or what is coming as your next move! Many protocol can be extremely useful and yes, there is more "must have" than the maximum open slot you can have so activating the right one for your need is crucial. On the downside, I must say that there is a few protocol that are almost useless... not completely but almost. They all could be use to fit a specific need although some of them you will either never use or just once every... couple of months to fit an out of ordinary need. Also the game suggest you an upgrade path which imo should not be followed (just like the contract or building upgrade). Every commander should look at it carefully before starting any upgrade. On the good side, it helps any active player tremendously given the fact that you can activate it every 3 hours. It is very good for new active players who just joined because, in all honesty, they have a long catch up game ahead of them in order to seriously compete
There you have it ;)