You'll get lots of recommendations, all very personalized.
Have fun seed-starting! I'm about to dust off the lights and the heat mats myself. Note: for tomatoes (and peppers and other high-summer heat-lovers), start seeds on heat and keep the seedlings on heat, or they'll just sit there and look at you. Start your cukes in peat pots or other biodegradable containers - their roots are delicate, and they "resent" transplanting from a hard-sided plastic seed tray.
I order mostly from:
Sow True Seeds (local peeps)
Botanical Interests (best seed packets for information, hands down)
Johnny's Selected Seeds (pricey but great quality and variety - they grow for the market garden industry, so if all you want of a particular variety is one seed packet, order early, or you'll have to wait until they've filled orders from commercial growers. Unless you want 500 vines of the same tomato plant,etc.)
Baker Creek Rare Heirloom Seeds (free shipping! wide range of heirloom plant seeds)
When I can't find a particular seed I'm looking for from the first four, I order from:
Totally Tomatoes (not my first go-to, but they carry quite a variety, and I've found seeds there that I couldn't find elsewhere)
Urban Farmer (ordered from them for the first time this year - nice selection, great customer service, but you really have to smack them down to keep them from emailing you to death)
edited to clean up the comment on ordering from Johnny's