I felt we needed a flip side to the "Plants You Love to Hate" thread. You can list by genus, species or cultivar but please add at least a word or two about why you like them.
My list would include:
The genus Paeonia - I've just started to explore the species peonies. Nice foliage, frequently great fragrance, longevity and nice blooms. I tend to prefer the singles, anemone/Japanese, and semi-doubles to the bomb and extremely doubled forms.
Nepetas - As far as I can the garden forms are a taxonomic mess and frequently mislabeled in the trade. 'Six Hills Giant' flops for me but 'Walker's Low', 'Blue Wonder', 'Dromore', 'Little Tich' and 'Dawn to Dusk' (huge, not a floriferous as many but still nice) have all been great. I just planted 'Candy Cat' so the jury is still out on this one. Sustained bloom and very undemanding. Does anyone here have experience with 'Joanna Reed', 'Pool Bank' or 'Sweet Dreams'?
Geraniacea - I'm not a fan of Pelargoniums but there are a ton of good plants among the true Geraniums. I love the foliage scent on many of them, it makes them a pleasure to work with. The much hyped 'Rozanne' has been a little unreliable for me here in Zone 5 but it certainly is floriferous.
Polygonums, Epimediums, Thalictrums, Iris (excluding modern TBs), grasses, sedges and ferns are all on my list too.