Overrated/Underrated

my only opinion applies to RCA interconnects

Underrated- MG Audio Planus (silver).  
I never heard any buzz, nor have read any reviews on these, but I was home demoing a preamp, and needed an extra set of interconnects to make the demo easier for my home set up.  Dealer loaned me a pair of these.  Their affect was more dramatic than the preamp.  Ended up passing on the preamp, but have now purchased multiple sets of these interconnects, replacing all others in my system.

Unbelievably (the right word) underrated (& this may make no sense until you’ve heard them in this way) is the 1970’s JBL 2500 bookshelf speaker (used & often available for 50 to 75 dollars) - PROVIDED you put in a kick-ass capacitor in the xover (a top WIMA in the pair I heard). Replacing the wiring helps too. It then sounds like a really, really good $1000 plus (maybe 2000 plus  or 3000 +) pair of bookshelf speakers. No bass to speak of, but transparency & musicality that are authentically jaw-dropping. The titanium tweeter is honestly better than any rebuilt titanium tweeter I’ve heard on any Wilson. The midrange driver is surprisingly, quite nice & detailed beyond all expectations in this context with good equipment, but overall no Watt. Someone did it as a lark & I was well, astonished. I’m no modder (I’m not good with my hands) but this I GUARANTEE. Highest quality parts are critical here. Add a sub & you’ve got a budget system for a relative, gf or someone else, that will make you look like a hero. For good reason. The other speakers in the range (2600, 2800) do well with this done to them, but this speaker has the best midrange driver. If I thought this could come back at me, I’d never suggest it here.

After auditioning 100s of cables as a cable beta tester. High Fidelity cables wins (loses) as the worst cable in every system I've heard them in-awful. 

Funny how some lines of higher priced cables results in worse sound.

Best underrated cable=Grover Huffman cables (under $900 ICs, A/Cs, Coax, etc. for the premier Pharoah line, half that for the Empress line).  Speaker cable commensurate with the length.  Heard many fine systems with under $1000 cables at shows.

I love me Maxx II’s and my Ev 600’s. Have had each for around eight years and have no plans to replace them

Overrated - too many to mention
Underrated - DH Labs BL-1