I've been in Thessaloniki just over a year now, so we've pretty much found our favorite places to eat here in the center where we live. I figured I'd be kind enough to share it with the world, so if someone googles "where to eat in Thessaloniki" they might get lucky.
Οπτά Πυρί (Opta Pyri), beside the fountain in the square behind the OTE building. This place has great food of all kinds, but I especially like the κολοκυθοκεφτέδες (mashed fried zucchini in a patty). Reasonably priced.
Ούζου Μέλαθρον (Ouzoo Melathron), right off of Venizelou near Tsimiski. This place has a kitcshy, fun feel to it, although the menu might be a bit daunting for folks unfamiliar with Greek fare. Also, the names of some of the dishes might be considered offensive by some prudish, George Bush type people. Also reasonably priced, maybe a bit more expensive than your average taverna.
Favorite upscale non-traditional, German-Greek food:
Extrablatt, on Alex. Svolou. All types of sausages and meat dishes. A plethora of beer choices. Interesting artwork, nice atmosphere. A little pricier than most, expect to spend some money on your meal here, but it is well worth it.
Favorite gyros/souvlaki/"fast food":
The Best, Ag. Dimitriou 146, tel. 2310.200198 Their tzatziki is quite literally, the best.
15 Grill, tel. 2310.870093
Luigi, Strat. Kallari 1 & L. Nikis, tel. 2310.240096 This place has the best burgers in town, and an Arabic pita sandwich that is quite delicious.
Favorite "home" cooking:
ΜΕ ΝΟΥ, Egnatia 95, tel. 2310.287848 Moussaka, keftedes, pastitsio - all the good homemade foods.
Favorite pizza and pasta:
Preludio, Justinianus 24, tel. 2310.252046 Oh my god, their pizza is SO GOOD. Their crust, the sauce, everything. They must be a little bit Italian - or from New York. And at 5 to 7 euros a pizza (depending on size and toppings), you just can't beat the price.
Since we live in the center, we haven't had occasion to venture forth more in the city, but if new delicious places tempt our palates, I'll post more.