It sort of depends on what your gonna play. If you've got a Stafleet campaign in the making, SOM would help. But if you want to add different species you should probably go for Aliens.
I'd advise Starships, but that not even on your list :-)
I don't have creatures but I think I'd say Aliens...
As a general thing I would say Aliens, because it fleshes out several of the major races and often provides a sample character to use (on the fly) for allot of the races in it.
Creatures is a nice addition to the collection BUT it's only so useful if you want to run Startrek meets D&D and have wandering monster encounters - the narators guide provides some quick and dirty aliens and their rules so you can design creatures with that allone.
Of all the suplements I have to aggree with Cut that the Starships book is probably the best, because it creates so much more than the basic rules.
I also (unlike some) quite like the Operations manual; it's small, but full of all sorts of tidbits, such as new fighting manouvers, additional professional abilities for your crew, details on fleets and a couple of extra races, which are fairly decent.
I just bought them as they came out, so it's not such a scary thing to consider bying them as a block!
Yep the starship book would be nice, but unfortunately it is not easy to find here for now. The Federation Manual and Aliens seems to be your favourites, I'll stick with that. I admit my question was somewhat flawed, because it depends on the campaign of course: we'll play the Enterprise Era, federation starship (and crew) boldly go blablabla. The stories will be focused on science and discoveries , with rather rare fights. Thanks for your help.
In which case the Aliens book might prove more useful, because those 'regular' aliens they list there may become YOUR aliens of the week - aka Andorians and Tellarites are rare and exotic to your players. In the case of the operations manual there's alot of info in there, about Starfleet, you won't need.
I can also recommend some of the netbooks on Owens' 'Memory Alpha' site - http://www.coldnorth.com/memoryicon/netbook/netbook.htm
Which contain some enterprise Era information (The 'enterprise Source book' and 'A Time for heroes' Pre TOS book) Which are for ICON rules, but the textual information should prove informative, and helpful. - which are also handilly free!