I don't know if the nimzowitsch defense is underrated as it has slightly bad stats, but checking my games out this morning, I haven't seen it ONCE in about 2,000 games as white. It used to drive me nuts as a 1400
The Top 5 Most Underrated Chess Openings!

Who is playing the opening though? White or black? On what move is it that the opening is officially the Czech Pirc?

I personally like the colle-zukertort.not sure if its over or under rated but I haven't seen it played.

budapest gambit isn't anywhere near drawish if white knows his stuff (source: i played it myself)
alekhine's defense, well, most club players seem to know about it these days. When I played it the four pawns attack was seen as challenging, but is now seen as a borderline refutation. I guess you could call it underrated.
1.Nc3.... lol yeah agree
icelandic gambit hard disagree. Langheld gambit from the from's gambit is probably better than that. I don't think this opening is underrated at all. I think a3 should take this spot. You can transpose into many black openings with 1.a3 where the extra move a6 would be helpful. I've played this otb with good results.
czech pirc... certainly not worthy of number one. it's just a worse philidor transposition and more committal to the pawn structure. Instead I would suggest the norwegian defense:

By the way, I made a huge course on the Czech in German last year. The English one -even better- will be released in mid-June by Chessable. The engine says notoriously +1 in the first couple of moves but actually I have the better prospects in the early middlegame. I have more short wins with the Czech than in other openings together (including white).

I saw my post from a year ago but just had to change it. I do like the OPs list as well as a few others here.
These are from my repertoire so I must believe in them -
1. The Wormsnake - this almost-never-played line is my main line for white nowdays - either 1. d4 d5 2. Nd2 or 1. d4 Nf6 2. Nd2 - it can transpose into the Breyer Slav, French Tarrasch, or the best line in the Colle. Otherwise its unique lines are quite sharp and favorable for white. It looks like a worm, but is infact a vicious snake which can uncoil with c4 or e4, or sometimes both, depending on which line you play. Often Qb3 gets played too, and sometimes g3 / Ne5. Most of the lines are actually quite sharp and tactical, c4 or e4 can come at any moment and with Qb3 things can get quite interesting.
2. Two Knights sicilian - it's the most complex transpositional sicilian setup which also happens to be almost never played... for a Van Geet player it's a must play -
3. van geet - there are so many transpositions in this opening, and it's so rarely played, that the majority of the time your opponent will end up in a mainline position they never play. Yesterday I think my opponent ended up in an exchange philidor by move 4... no one wants that.
4. slav gambit -
If white responds correctly, which people rarely do, it's still a completely fine position for black that the engine likes as much as any other black opening. If white screws up, which almost everyone does, black usually gets quite a large advantage as he just retains the pawn and has great development / a complex dynamic position.
5. nimzowitsch sicilian -
I've been playing this sicilian for about a year. I think I've gotten the mainline "refutation", where black is down a pawn but has dynamic compensation and scores well in practice... maybe 3 or 4 times out of hundreds. About 1% of games at 2200+ elo reach the mainline position. The other lines... are just very confusing for white.
UGH!!! effing android toy computer wouldn't let me escape the quote after i edited it. If BDG is unsound, how did the 2200 who put me on the path to playing the gedult via the scandinavian move order beat deep blue with it in a blitz game, and how do i have winning stats with it, CRUSHING stats with its mieses carokann cousin and the ONLY close to equal stats against the french with alapin diemer?
BDG has something like an 8% edge 1600-2000, and even Levy and Nakumura rank it as "unbeatable". There's more to effing life than a single pawn.
As a king's gambiteer, gedult BDG gives me my EXACT preferred tactical tools... knight on f3, bishop on c4 and a juicy semi-open f file that punishes positional players for their lack of tactical smarts. I know this because I BEAT the BDG as black by a few % too and it isn't because of booking up. It's because I'm more of a tactician than other tactical players. It's all I know and do.
I don't listen to elitist GMs or their amateur parrots. They are MOSTLY clueless about what happens outside their pawn pushing bubble in the REAL WORLD where tactics BEAT position.
Look at any LEGIT gambit, and you're likely to see a plus 8% stat 1600-2000 where NO ONE is consistently 95% accurate and its common for BOTH players to have 50% accuracy in a game.
Based on THAT stat, I'll concede that gambits win in the battle of inaccuracies which is another way to say they're MORE accurate in amateur games.
If they're as bad as haters say they are, yet having winning stats, then haters are worse
Alekhine doesn't bother me one bit. I just don't play into its knight chasing PLANS. I have winning stats with both a line I forgot AND returning to the krejcik. Wreck an alekhines castle on move 3, and it's on