There is no general answers for this - this depends from your personal workflow and the tasks you work with - but it is mainly independent from which music you work on/with.
There are many which tell you UAD2 systems are "expensive dongles", because you get some of the plugs even on native intel / x86 too. This is far incorrect. The UAD2 systems are build on high performance DSP plus FPGA "stones", allowing a much higher throughput of audio data through complex signal processing. This means: the UAD system is in his home area, where it can provide huge "processing power" by a plugin. Afaik any UAD plugins runs internally with 192 kHz (upsampling) - even if you use a plug which is available natively (i.e. brainworx/plugin alliance). This may lead to different results. This means, that UAD could play the most out with plugins which would require huge amounts of DSP power on native x86 systems.
There are not "must haves", but some of the plugs are very widely used within the pro league att.:
- Manley Massive Passive EQ
- Neve Mastering Compressor
- 1176 suite (the new)
- LA2A suite (the new)
- Pultec EQ Suite (the new)
- AMPEG / Studer Tape (suite)
I would decide for ONE of the channel strips available (API, Neve, SSL). I started with the API Vision console, which brought me a very new way to CS mix - even if i use it only on some channels today. That beast sounds very musicial and nicely analog, but the handling is a bit "new" for first..
This should be a "complete" comfortable set for a UAD "beginner". After that, try to use the demo period of plugs one by one (and hear the demos) you might be interested and/or read what others recommend for special tasks. You will know what tools you need further when you at a stage where it makes sense to buy it.
The most UAD can play out are plugs with very complex impulse responses from analog equipment, but UAD still has some very nice "emulations" (or "1to1" implementations of) well known studio gear - i.e. Reverbs, distortion/harmonics stuff etc..
For small studios the "Ocean Way" plug is very nice to bring the sound of a pro studio into the home or semi pro one. The Lexicon is a nice alternative to buying a hardware one.
There are even some plugs which seems not so near to the original and got some complains from several users (read the comments on the plugs in the store!) - i.e. the Shadow Hills Mastering Compressor is very far from the "quality" of the Neve Mastering Comp. - but is still very musical to use (means: if you don't look for a "software version" of the hardware...).
Use the demo period and try the stuff - do this early enough to get a plan what you want to buy when UAD has a harder short time sale (around end of the year) where you could get up to 70% discounts. Very new plugs usually seems lesser discounted for the first year.
There are even plugs which are so light onto CPU on x86 too that they are no "must have" on UAD, but could be a nice alternative if your PC / Mac is under heavy load. I prefer to buy that only when heavily discounted - the cheapest one (native or UAD).