Helen, for starters get yourself ANY version of a 90/2.8 Elmarit (they are all great, each generation with their specific different character, the latest being optically the most perfect, earlier versions having other nice traits, as lower contrast or super-small size like the thin Tele Elmarit or a lovely smooth character, beautiful for portraits as the v1 or even the now luxurious short-fat Elmarit).
If you can splurge a bit, the latest pre ASPH 90/2 Summicron is my favorite for pure beauty - its a true Mandler lens perfectly matching in character with the other great Mandler classics + it is even in performance with the contemporary latest Elmarit-M from apertures ~f3.4 - closed down if you really want sharp and contrasty now and then.
It is also barely larger and heavier.
The smallest and probably most pocketable 90mm of them all is the 90/4 Macro Elmar.
This is a really modern, high performance lens with super crisp, sharp details, high performance close up (Macro with adapter) and even at infinity.
Unfortunately this once overlooked lens (many aperture snobs sniffed at it's slow f4 opening) has picked up in price extremely - once a secret tip it is now hard to find a nice sample for anything less than the mandatory 2000 USD wall :-(
Funny, how people who dismissed this lens for it's slow aperture overlooked that it actually has some the smoothest, nicest and most beautiful out of focus rendering!
For absolute best bang for the buck these would be my two favorites:
90/2.8 Leitz Elmarit v1 (M mount with vulcanite strip and made of a mix from silver chrome brass and silver anodized aluminum)
85/2 Nikkor LTM (super heavy, yet very compact built from massive brass and durable silver chrome - there are a few very rare black chrome samples around but these sell for 4-5 times the price of the otherwise identical regular silver version).
The Nikkor is a classic Sonnar design and is in imaging character very, VERY close to it's rare, big, chubby sister the 85/1.5 at aperture f2 and smaller - best deal in classic ~90ish lenses if you ask me. As a Sonnar lens it needs some initial adjusting to your taste by a good tech and will perform best wide open for a small distance range you specify (it can't cover the whole distance range from close focus to infinity wide open with best performance as of it's Sonnar design but will be great where you want it).