There are
lots of good speargun options. I currently like
Omersub (It), they seem focused on spear fishing and continually innovating & refining their product range. I use the super slim, super light Omer XXV - seems v. well suited to the bass & mullet we mainly get in the UK - but I like the look of the Cayman models too.
Beuchat (Fr) has a good reputation (I don't like the look of their cheapest models but they have stood the test of time - their newer models like the Marlin look very good).
Picasso (Esp?) (not keen on their cheapest models but others look ok).
Dessault (Fr) - for those that want something a bit different, said to have the best rubbers by some (Spaghetti

).
Seacsub (It),
Cressi (It),
Seatec(It).
Railguns seem like the Landrover Defender of the speargun world to me: big & tough perhaps not the most refined or sportiest models but proven and evolved over time, able to handle the biggest challenges but can sometimes feel a big big/cumbersome. Railguns able to handle big game fish:
Mako (see the owner's posts here on DeeperBlue, TW/US),
Rabitech (
sumora, official distributors of Rabitech, TW/SA),
Rob Allen (SA),
Hammerhead (HI, USA). Rails are not essential by any means, generally considered to be only of real benefit on barrels 100+cm - a more important feature of most railguns for me is the soft Dyneema wishbones, which are simple, safer, quieter than there metal counterparts (but less long lasting) - and the heavy-duty rigging if you plan to go after
huge fish.
Woodies:
Riffe (CA, USA),
Darryl Wong (HI, USA), JBL/Billing (if they float your boat), etc.
Then there are all the small custom builders - I guess Darryl Wong would fit into that category too. Plus several in the USA (Setting Steel?), Italy, Greece and...of course...Guernsey

.
Or you could make your own, as several forum members have (including Foxfish several times, Magpie & Portinfer - all on Guernsey). Check out the various discussion threads on this forum for more info. on that option.
Cressi Commanche used to be considered a good value speargun to start with. Good enough to be used by an Italian World Champion but reasonably priced (in most of Europe) & with excellent spare parts availability. Bit plasticky for my tastes but I think they look pretty (the raked handle, etc.) cool & work well. They have some interesting newer models too.
Omer Excalibur too.