So should preface this is going to be completely biased based off of my opinion and aircraft I have actually purchased and flown enough to give a rundown. Aim is to give a list of aircraft that are good buys and which ones to avoid so hopefully newer players to online can avoid purchasing mistakes. If you have a different opinion feel free to chime in and in fact it is encouraged. I'll be basing my opinions off the coop modes and look to try and breakdown positives and negatives.

These aircraft are excellent purchases and are a solid investment.

F-4E
+Extremely cheap to upgrade and quick to level
+With UGB it makes a great early ground attacker
+Quite effective in Emergency Sorties
-You basically need to level it at least once to become competitive

F-14D and MiG-31
Putting these both together as they share the same positives although if I was to pick one over the other I'd say the F-14D just because of access to GPBs does come in handy.
+Quite cheap to purchase and level up
+Doesn't need to level up to be competitive, LAAM allows you to quickly rack up air kills
+Very low skill ceiling, easy to fly essentially
-They're not that good in Emergency Sorties

Mirage 2000-5
+Excellent ground attack aircraft despite being multirole
+Good abilities
+Doesn't need upgraded to be immediately competitive
+Can hold its own in a dogfight
-If your intention though is to fight aircraft there are better options on the table

Su-34
+Excellent ground attack aircraft that actually has the speed to get to a target
+UGBL devastating against ground
+Can hold its own in a dogfight
-Somewhat expensive especially as you are going to want to upgrade it straight away

These basically have so many caveats that being good or bad is mostly up to the individual.

ADFX-02 -PIX-
+In the hands of a skilled pilot this is a great plane that can dominate especially if you are good with the TLS
-In the hands of a bad pilot this is a wasted aircraft and you are better with something else
-Expensive to upgrade
-Paid plane

F-14A
+Cheap to upgrade
+Surprisingly effective in Scrambles especially with SAAMs
-Paid plane

F/A-18E
+If you are good with AAM and SOD this is an excellent aircraft for you
-If you do not like these special weapons get something else

These aircraft are poor investments typically due to there being much better alternatives out there.

A-10C and Su-25SM
Lumping these two together as they basically have the same problems. I cannot think of anything positive to say about them either.
-Too slow
-Poor loadout options
-Outclassed by most aircraft including early multiroles
-Not even that cheap

F-2A
+Quite strong in Emergency Sorties
+Reasonably priced to purchase and upgrade
-Other aircraft performs its role better
-Doesn't have the best of loadouts

F/A-18C
+Cheap to purchase and upgrade
+Surprisingly capable in Emergency Sorties
-Outclassed by other aircraft
-Better off upgrading the F-4

MiG-21bis
+Cheap to purchase
+SASM makes it handy in a Scramble
-Too slow to be competitive in other modes
-Not that cheap to upgrade