A true tactical light fits these criteria:
1. Single output - no strobe, no high/low. Just on and off.
2. Momentary switch - press for on, release for off. Twist for constant on, untwist for off, untwist farther for lock out.
All other aspects of the light (brightness, battery type, materials, etc.) can vary. But those 2 criteria listed above are what make a light truly tactical.
One of the best tactical lights you can buy comes in 2 separate pieces.
Piece #1 would consist of a Surefire made of aluminum, with a glass lens, taking 2 CR123 batteries, with a Z41 tailcap (press-for-momentary/twist-for-constant), and a removable head. The models that fit these criteria are/were: 6P Original, C2 Centurion, and Z2 CombatLight. The C2 and Z2 both had rubber grip rings. Both have been discontinued. The 6P Original is still being produced.
Piece #2 would be an M61 drop-in LED module from Malkoff.
Other good options (if you don't want to piece together your own light) that still fit the 2 criteria are Surefire's newer tactical lights such as: G2X Tactical, 6PX Tactical, G2Z, G2ZX, and Z2X. The downside is that these newer lights either have heads glued to the body, or heads with the LED module sealed inside them, so they are not modifiable. Some people don't mind this.
My personal choice is a C2 Centurion body, with a 6P Defender crenellated bezel, running a Malkoff M61 HCRI (high color-rendering index) drop-in.